That's the fastest I can imagine, though I also expect that the administration will need to expend a lot of incremental political capital (and likely fail) to make the cap gains change affect 2021.
More likely, again MHO, is pass something in 2021 that starts with the 2022 tax year. Getting agreement to lower taxes starting in the current tax year (or more down than up) is pretty easy. Getting agreement to change the tax laws in this big of a fashion and have it affect the current tax year (inconveniencing, at best, people that were relying on the then-current tax laws in effect for their tax planning) is a bigger lift. Any bets on a flurry of lawsuits to stop a 2021 implementation as well?
Getting a tax increase passed that will affect the tax year in which it's passed sounds to me like a really tough incremental lift. Specifically because planning and action for that change would have to happen right now when there isn't even a real proposal on the table - only a campaign slogan / promise.
Even more likely, again MHO, would be something passed in 2021 that phases in part way with the 2022 tax year and phases the rest of the way in with the 2023 tax year (i.e. - going the other direction).
Real answer is I dunno - I'm just guessing and spitballing. But the optics of trying to change the 2021 tax year by raising cap gains tax rates - that's just too easy for naysayers to take the idea to the woodshed over an implementation detail and have a reasonable shot at scotching the whole thing for the implementation detail. That sounds like bad strategy to me.
I do agree with your observation about how quickly this could happen though as well as your conclusion given the likelihood that somebody assigns to the possibility.