The reason to be nervous is because the electoral college tilts in the GOP's favor and every presidential election is close. Further, the mainstream media will normalize Trump (again), play up Biden's liabilities, and do everything in their power to manufacture a close race.
But you're right, and that's a huge list of negative factors the GOP hasn't ever really dealt with in a presidential election year before. The state GOP multiple battleground states has very quickly become about as effective a political machine as the Democratic party has been in Texas for the last 30 years. And Trump's trials will not help him with undecided voters (and his defense will also peel a ton of funds away from other candidates).
Polling this far out is always pretty much worthless, and polls have consistently missed the anti-Dobbs backlash that we've seen in every election since. Worries about Biden's age aren't going to negate that, no matter how much some centrist twits in the media want it to.