I think you may be greatly underestimating numerous factors and reading too much of your own sides messaging. This notion that the Dems are the only one with a stable majority is delusional.

For one thing, 4 years of Hillary, who is barely supported by much of her party, after 8 years of Obama, will very likely add to Dem voter malaise and apathy, meaning a low voter turnout for the Dems in 2020, which benefits the Republicans. The American political system is a pendulum, and people swing back and forth between parties. One party hasn't held the white house more than 3 consecutive terms in nearly a century. Hillary, if she wins, will be like Daddy Bush's one term after Reagan.

Secondly, I suspect you are vastly under-estimating the support for the current, alt right, trump style right wing politics. As America diminishes in terms of an economic powerhouse, that angry white male vote becomes very strong, and even moves beyond just angry unemployed rural labourers.

Something the current American left refuses to acknowledge is Romney almost won in 2012, even with a deeply divided base. A uniited Right base, with a divided Dem base would be a very very easy path to the white house. Anyone who tells you the Republicans are done is just selling you what you want to hear.