The problem for Republicans is Texas. Democrats were rightfully disappointed that Texas wasn't closer this year, but zoom out a little and the trend lines are still looking good! In 2012, Romney won by 16%. In 2016, Trump won by 9%. This year, it's going to be around 6%, with unprecedented turnout. If Biden is a successful president, and Texas continues to urbanize, and turnout returns to the mean, Texas will go blue--by 2024 possibly, but certainly by 2028. Once that happens, how do Republicans win the electoral college?
Republicans need to nominate someone who threads the needle between firing up the Trumpy base and not alienating the suburbs. I'm sure there's someone out there who can do it, but the person who best fits this description was pre-Bridgegate Chris Christie.
They should also hope that Democrats nominate someone unpalatable, which is definitely not out of the question. Given how close the election ended up being in the critical states, it's possible that Biden was the only candidate who could have won this year.
All you say is true, but it's hard to see how either of these things actually happens. For obvious reasons, I think Trump had unique ability to fire up the Trumpy base that we're not likely to see imitated for a long time to come. He had been a prominent cultural and media figure for decades before entering politics, with a deeply ingrained (and false) image as a genius businessman and powerful leader. Who's out there who could pull that off again? As for the Democrats, Harris will obviously be the nominee after Biden, and given four (or maybe eight) years as a competent VP she'll be rightly seen as a standard, palatable candidate perfectly acceptable (if not preferable) to a majority of voters.