2016 was a weak GOP line-up, look what happened.
1972 was a strong line-up, look what happened.
1988 was a strong line-up too (besides Dukakis and Jackson anyway), look what happened.
In short: Wait and see. And if Trump manages to stay where he is, then it'd take some kind of anti Hail-Mary for the Democrats to snatch defeat from the jaws of victory even with someone like Kerry or Dukakis or 2000-era Gore anyway.
I'm a cynic. I see Trump as a nigh-unstoppable force. I have relatives who are hardcore Trumpists, and Trump can literally do no wrong. These aren't outwardly racist Klan type people, mind you. But Trump to them is how FDR was to a lot of people in the 1930s - viewed as akin to a savior. I've seen (non-ironic) comments saying Trump is the best President we've ever had.
The fact that a lesbian married couple I know in PA are hardcore Trumpists doesn't really help my faith any. Even people who would be out of step with GOP values - like a 26 year old former Heroin addict I know who is into partying hard with hard drugs even now - loves Trump. These aren't the kind of people you'd have seen rallying around GW Bush.
Why do these people love Trump so much?
Lesbian couple: Because he's not a politician so he's not corrupt.
Former addict/party girl: https://www.facebook.com/An0malyMusic/videos/1617404181603525/?hc_ref=ARTUkjvnv3jX83R70N6SiWceY7Wq7viUBAZSCR0XSwPRnlNK_zXlxJMcG5gAEbPhhUw&pnref=story.unseen-section
That's a video she reposted.
This is probably the unreachable type who will just see what they want to see and ignore everything else. I've heard a good number of people who voted for Trump say they only did it because Hillary Clinton was too unacceptable.
Of course, this doesn't mean Trump is DOA in 2020. He was underestimated for an entire election cycle and look what happened. Like was said earlier in the thread, a good bench =/= wins, though the Dem bench seems fairly strong right now. Most of the most commonly thrown around names I think will be too old to be serious contenders, or too inexperienced. Besides, the Democrats need some fresh faces who don't have universal name recognition right now. The 3 strongest potential contenders I hear are Klobuchar, Gillibrand, and Bullock. Klobuchar has history of getting a lot of crossover votes (see her latest reelection results) and could excite the more progressive base. Gillibrand is in a similar boat, but might rely more on the progressive wing. Bullock would have the advantage of not coming from Washington, and has a remarkably progressive record for being from Montana, but has a mixed record on gun control and renewable energy that might turn off progressive purists.
Poll
