Yeah, then a recession hits and we have a new Republican generation after the millennials. Don't get your hopes up.
Obama had to endure a lot of blame for the Great Recession even though people do associate the start of it with Bush. Most recessions are not like that one. Reagan had to deal with a recession too, but it didn't stop the conservative tide that swept up that generation. Recessions seem to happen almost every decade, so it's not something that will overwhelmingly hurt a party in power long-term, unless it is a massive recession. It may cost Democrats an election cycle or two, but what the parties stand for and how closely the people align with them matters arguably more. Nixon's catastrophic implosion hurt Republicans
big downballot from 1974 - 1976, but they bounced back rather quickly because the public still favored the party and what it stood/pushed for.
As it stands now, Millennials are going to remain a strong Democratic generation and based on the GOP's current position, the next generation is not likely to be significantly Republican. Most of history shows either balanced party support or slight advantages for certain age groups, and only every 15 - 20 years or so do parties pick up major, lasting support (Eisenhower/Republicans, Nixon/Democrats, Ford/Carter/Reagan/Bush1 - Republicans, then Clinton/Bush/Obama generation is pretty heavily Democratic).
Finally, even if the next generation is fairly more Republican, it will take a long time for that to trickle down to Congressional races due to the low participation rate of young voters in midterms. That's how it has always seemed to play out. The pendulum always swings back, so yes, Republicans are going to gain an edge eventually, as they have before.
It would take Trump taking office and destroying much of the US for what you described to occur.
You mean Democrats taking back Congress in the 2020s? That is already going to happen, imo, regardless if Trump becomes president or not. It probably will
not happen as long as a Democratic president is in office due to the issues of making legislative/Congressional gains when your party is constantly being blamed for the country's problems, as is typical for the president's party. The exception I think is that we might get a Trump wave this year, but will probably lose a lot of those gains in 2018.
I think the trigger point would be something akin to Bill Clinton's 1994 disaster. That wave was always going to happen due to the buildup of Republican-leaning voters that were beginning to outnumber Democrats in midterms. The same will happen for Democrats at some point.