That's my gut feeling.
I don't think it's at all impossible for Biden to win, but that's my gut feeling.
To be more specific: I think the "Red Mirage" scenario will happen. Trump will have a lead on election night and he and the Republicans will pull some fuckery to stop mail-ins from being counted in the right places.
If it gets to the Supreme Court, either Biden and the Dems will pull a Gore and surrender the election without a fight, or the Court will rule in his favor and hand him victory.
History will repeat itself.
This logic only applies to mail-in ballots that arrive after election day, though. The vast majority of early voting was in-person or mail-in well before election day, so it will get counted along with day-of in person votes just the same.
It should also be noted however that for the past several months Trump and the Republicans have done their damnedest to fuck with the entire mail-in system and make it as broken and inefficient as possible.
We'll find out soon enough whether or not they did enough damage to make up the difference.
What they fucked with was the USPS, not the way mail-in votes are counted (generally, they did get a few court cases here and there to limit mail-in counting). And I don't have numbers for this, but I'm pretty confident most of those mail-in votes have already arrived, and there are way more in-person early votes than mail-in.