I'm not sure, so everything next is more like my opinion, not their reasoning.

In theory system of choosing yourself a role before queue is good. Less "toxicity", less fighting for lanes, especially important for LoL, which is a bit less flexible on this matter. Another reason why they made this system, it's because it gives them more players. In this system it sucks for player to play alone, so by their expectations he will take his friends and pull them into the game, because he need someone to play with.

Also you can see signs of this in their another new thing, Hextech Crafting. Basically it's the way of getting stuff like skins by playng. So, you have these boxes, which you can get from the store or by achieving masteries. And keys for them are dropping in the matchmaking, but ones who are playing in the group have drops more often.

But in reality I think that Riot a bit detached from their playerbase, at least from non-casual one. They are like Blizzard in this moment, thinking that they know better what players need. They are trying to justify their decisions by empty words and stats(sometimes not demonstrating it, so it creates impression that they are pulling it out of their ass), while leaving the game without really important stuff. Game doesn't have replays, you need to go to 3rd party services for this, it doesn't have voice com, because "it increases toxicity", current client is fucking terrible, and new one is in development for years, remake in case of disconnect of one player was added in the game this spring(!), it doesn't have damn pause function. And all this toxicity crusade is quite ridiculous too.

Moment with sandbox was a good demonstration how detached they are and how they love to bullshit. And yes, shitting on solo players might be profitable in terms of new players, but it also makes them more frustrated.

So this system might be good in some aspects with a lot of polishing, but bullshitting by Riot made things worse. If they didn't though that they know better, if they didn't lie, if they didn't force players on one thing, but gave a choice, it would be fine, more or less. But I'm sure that in this case most of the players would never play dynamic, and that would show that Riot fucked up, and they really doesn't like to admit that they fucked up.

Like Gaben once said:

'Don't ever, ever try to lie to the internet - because they will catch you. They will de-construct your spin. They will remember everything you ever say for eternity.'