Biden was ostensibly too old when he became President, but he hadn't started the obvious decline that we all witnessed in 2024.
And I still would much rather have had Biden for those four years than have had to face what we're facing now with Trump back then. Except with Trump being 4 years farther away from death than he is now...
We did have that discussion and almost certainly the DNC also had it with more than a handful of media appearances written off as gaffes. They decided their best chances were to stick with Biden.