I expect this post will receive significant challenge, but I'm commenting in good faith. My comments here are more practical than substantive. Please note that I am posting links to some studies/papers not with the belief that they rebut all of your premises, but simply to show that not all of this is a foregone conclusion and there is some debate as to the effects of diversity.

There's nothing inherently wrong with a homogeneous society, though I expect it would become stagnant, and weak on the world stage - regardless of the ethnicity of the inhabitants.

However, there are two (related) major obstacles to the creation of a homogeneous white nation. Both are practical in nature, and essentially boil down to the fact that creating a country out of an existing one is very hard, and most people won't support it.

First, no first world or G20 countries are ethnically homogeneous, unless ethnicity is defined so broadly as to become meaningless. I specify first world or G20 because these are the kinds of places where white nationalists are calling for ethnically homogeneous nations. There are no major ethnically homogeneous white nations at present. The bell of diversity has been rung across the western world, and it cannot be unrung. Wherever there is even a tiny minority in a country with constitutional guarantees of equality, there are serious difficulties with homogenization.

This means that creating a white state would require either destroying an existing country (or at least having a meaningful part secede) or somehow exporting all non-whites (however defined). This will face obvious political difficulties.

Dividing up an existing country (including by secession without civil war) would require constitutional amendment, which will generally require a supermajority of the relevant legislature and/or the approval of the sub-national geopolitical divisions such as states or provinces. This is politically impossible at present.

Exporting the undesirables (even if purely by positive financial incentives) would also require constitutional amendment, as any such policy would be contrary to the bills of rights of the relevant countries.

In the USA, approving a constitutional amendment requires 2/3 vote of Congress and the Senate or a 3/4 vote of the state legislatures (I simplified the process, but those are the numbers). According to Wikipedia (boo, hiss) non-Hispanic Whites make up 63.7% of the American population. That's less than 2/3, and significantly less than 3/4. Accordingly, any constitutional amendment to eliminate the constitutional rights of minorities (which will be required in order to effect even the mildest of policies for the export of non-whites) will require the cooperation and votes of those minorities. This difficulty will only get larger as America continues to turn brown. Moreover, as I will discuss in my second point, many white people will also oppose homogenization.

The other option to create a homogeneous society - which would not require the cooperation of a supermajority of the country - is of course civil war. Um, good luck with that. That always works out great. Bear in mind that many people - including many white people - put national pride or patriotism above their racial pride or ethnic nationalism. Also bear in mind that military and police members of most countries swear oaths to defend their country, its constitution, and/or its people. They usually take these oaths seriously.

The second obstacle is that the majority of people in Western countries don't want a homogeneous society. Bear in mind that people may be anti-immigration but not necessarily pro-homogeneity - their concern is usually with outsiders entering their country, not with established minorities within their borders.

Note that more than 75% of respondents from each of the US, UK, Australia, and Canada (among others) indicated that their community was accepting of people from other racial, ethnic, and cultural groups (see http://www.conferenceboard.ca/hcp/details/society/acceptance-of-diversity.aspx but the data is derived from the Gallup World Poll). Most people in the western world accept diversity.

Yes, some studies suggest that diversity may harm social cohesion. Few people will deny that there are challenges associated with diversity (as with anything, including economic development, internal law enforcement, and national defence).

However, other studies (see e.g. Letki, 2007: https://www.nuffield.ox.ac.uk/politics/papers/2005/NLetki_social%20capital%20and%20diversity_final.pdf) indicate that socioeconomic status is more important, and that only those diverse communities that are economically disadvantaged have social cohesion issues.

Some research suggests that diversity accelerates technological and economic development and homogeneity slows it down in an industrialized society (see e.g. Ashraf and Galor, 2011: http://www.nber.org/papers/w17640.pdf). Diversity of businesses and their leadership teams is strongly linked to higher returns (see the multiple studies from McKinsey & Co.). The biggest factor is the exchange of different ideas and perspectives, which aids innovation. Innovation-based industries such as the technology, biotechnology, and pharmaceutical industries in particular love diversity.

Diversity appears to be a good thing from a technological and economic standpoint at least, and many people (particularly on the traditional right) believe that what's good for a given country's economy is generally good for the country and it's people. Many people (again, particularly on the right - see the common argument for Trump) believe that good business leadership carries over to good political leadership.

Economic strength enables all other kinds of strength. It is required for a strong military, to allow time and investment in cultural activities, and to allow time and investment in technological advancement.

Of course, I know that white nationalists generally prioritize preserving their ethnicity/culture/race over most other political considerations, including the economy. But what's the point of having an ethnically ideal country if it's shit aside from that? Myself and virtually everyone else I know care more about having a decent job, good standard of living, and living in a decent neighbourhood than the color of the skin of the people around us. You will note that a great deal of mainstream anti-immigration sentiment is premised on economic arguments (e.g. they're taking away jobs, or they're going on welfare).

What this second point boils down to is that the majority of people (on the left for primarily egalitarian ideological reasons, and on the right for economic reasons as well as egalitarian ideals) simply will not support efforts for a homogeneous society of any kind, because diversity (which is a separate issue from immigration) is generally seen as a net benefit.

TL;DR: Aside from civil war or secession (which would likely lead to civil war), efforts to create an ethnically homogeneous nation are bound to fail due to a lack of popular support (for economic reasons and by reason of egalitarian ideals - even if not everyone believes all people are created equal, most believe all should have equal opportunity and equal protection under the law). Moreover, as the trends of the past 50 years towards increasing diversity and increasing acceptance of diversity continue, popular support for a white ethnostate will diminish.

TL;DRx2: a western white ethnostate is a pipe dream.