Why would you say that? President Trump took 31 states. That's 62 Senators right there.
(The real window of opportunity was 2018, though, since there were far more Dem seats up for re-election. In 2020, there's far more GOP seats up for re-election.)
62 senators are not up for reelection. Only 33-34 normally plus any that were appointed in the interim. Most will be in safe seats.
I think lots of reluctant Democrats that will end up voting for Trump will end up splitting their vote for Senator as a check on that. Maybe I’m wrong.
People did not vote for Trump because he was a Republican. They voted for him despite the fact that he was a Republican. (A bit of an exaggeration, but this was certainly true for a lot of voters). Trump will never have much in the way of coattails, because the Republican party has done nothing to make up for decades of betrayal. Trump will win easily. But Republicans are on their own.