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, and he's obviously no longer viable.
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.