Well, one downside for the Democrats would be that a lot of their senators are facing re-election, too, and those that aren't would probably want to be out campaigning, too, whether in their home states or for Biden and Harris, or in other elections.
So, here is the thing - in 2018, most of the senators running were Dems, and the basic probability was that some of them would lose their seats to a republican [and at least one did because of a combination of then Gov cheating to get a seat, combined with 'progressive' purists voting against the Dem for our state because he wasn't progressive enough - thanks assholes, we ended up with Voldetor Skelemort, are you happy now?]
This go, if I'm not mistaken, the math is actually in the Dems favour - more GOP seats up for grabs and probability says some will go to Dems.
So, short of cheating/outside interference from foreign powers, the Senate is likely to flip. However, considering that they were trying desperately to make getting offered aid from outside nations something they don't have to report... well, I'm a bit nervous that 200+ years of cycles are out the window.