The other key point here (as @mattyglesias pointed out in his newsletter today) is that both "pre-existing conditions" and "essential workers" are highly amorphous categories that will be gamed (especially by the rich and powerful) whereas age is not. https://twitter.com/NateSilver538/status/1339260042378424322
At the very least, define these categories *narrowly* with some sort of empirical basis. If there are 87 million "essential workers" (as per ACIP) that's not a very useful category; that's more than half the working population in the US (~160 million)!
In New York City—which has good data on death rates by age—people age 75+ represent 6% of the population but *49% of COVID deaths*. This is a no-brainer. If your plan doesn't prioritize people in this group, you screwed up and need to start over.