"As I said in another comment further down, Biden's reliability (i.e., his reliable self-restraint in comparison to some lunatics (I would imagine) in the defence department) may be being compromised by the effects of old age."
Biden was compromised by his wealth and sense of ethics long, long before old age set in as a further detriment. He has always represented the exact same status quo as Republicans, just as all the current crop of Democrats do.
"However, the relative merits of Biden and Trump may be unimportant now as the next presidential race will almost certainly not be between these two."
Agreed, but if they were both running against each other, there would be no "merits" to either that would make any difference for the common person. The end result would be business (literally) as usual.
"It seems to me that voting for the lesser evil can be a rational course in general,"
No, it cannot IMO, because this is a con game used by the ruling class to keep us voting for the continuation of the status quo and business as usual over and over again, making us think our only choice is a "lesser" evil, which means we're still getting an "evil." Americans, or people in any nation, should never allow themselves to be trapped in such a self-defeating situation. We can take matters into our own hands as an organized class, and until we do that, the two main parties serving the exact same donors are going to continue bringing us more of the same.