The problem is that none of the nominees (other than Biden) will have been picked by a primary. If it's not Biden, it's someone who won't be picked by voters.

Neither was Harris. Heck, she never ran in a single primary, having folded her tent before they even started. (I realize this is a debate point, surely there will be *some* who are offended that she is passed over (particularly minorities, I would suspect), even if she is not passed over and has the same shot as anyone else in a brokered convention.)

Worse, it’s been a half century since either major party picked a candidate at a convention

True, but not persuasive. This is an extraordinary case. Should Biden opt not to continue, the option isn’t to automatically award it to Harris any more than it is to award it to Bernie Sanders. There is a process in place. Yes, it’s unusual since it hasn’t been used lately, but then it was used for around 200 years, so it’s not as though it’s just been made up on the spot.

Other countries don’t pick their nominees for head of government this way

I’m not sure why that’s important at all. First, 99% of Americans have no idea how leaders are picked by other countries, and likely don’t care. Second, anyone who has sat through a droning history class in school knows “conventions” were used. I’m confident that 99% of those are hazy on any details, but it’s not as though a genie appears and said “try this!”

The convention can’t replace that

And yet it did for the great majority of our history as a Republic. If Biden should withdraw, what would be novel is automatically awarding the nomination to someone who never entered a single primary in her life.