Bernie Sanders might win if he runs in 2024
He won't run for a variety of reasons, but if he did, the perception of age as a non-incumbent would probably make him a significantly weaker candidate than in 2020. At some point, it will become politically viable to run for president into one's 80s just as became viable in one's 60s and 70s, but the contours of the last primary season would suggest late 70s is the upper limit without seriously hurting the competitiveness of one's own campaign.
Baldwin or Brown might be able to take up the mantle, but like Sanders himself, they'll probably prefer to focus on their Senate races. They are also untested at the national level, so 'might' is doing a lot of work there.
Interesting governors like Inslee and Bullock struggle to break out of the pack because they don't have the media connections of CA/DC/FL/NY, which limits the pool of potential candidates in contemporary primaries.
The Gen X crop of Democrats does seem to be rather small and unimpressive relative to their elders and newcomers, some of whom might be ready for a run by 2024. I wonder if the seniority system in the House caucus/gerontocracy has contributed to the relative lack of talent in the middle.