Well, it may be that the Senate Republicans are waiting to see the results of Trump's death throes before deciding whether or not to work with Biden. But I'm not sure I agree it's *that* pessimistic. It would be 2014-2016 all over again, when Obama's agenda was frustrated time and again, except possibly even more so: at least by 2014, Obama had had six years to put some of his agenda into place.
It's worth bearing in mind, for example, that Biden's Cabinet picks all have to be approved by the Senate. Ditto any judges he nominates: in the SCOTUS, for example, three members currently are over 70, including 82-year old Breyer, who is one of the three remaining liberal-nominated Justices. If he dies or steps down, it would not be too difficult to imagine the Senate refusing to allow Biden to nominate a Justice as long as they are able, just like they did when Scalia died and Obama tried to nominate Garland, and just like they didn't when Ginsberg did and Trump and McConnell forced through her replacement in barely a month.
I don't have a problem with the principle of a Republican Senate frustrating any of Biden's potentially more "radical" policies, but I can easily see a scenario in which they frustrate as much as possible of what he tries to do. If so, the 2022 midterms could be particularly crucial to help resolve the stalemate, or cement it.
In short, I'm not optimistic that the next two years will be all that productive. As long as the Republicans are still taking their cue from Trump and, in particular, McConnell, I don't think I'll be optimistic at all that the US can return to compromise and cooperation.