population of people who currently sideload and don’t know about the deadline must be small

Oh yes totally. Misunderstood you there. This number is probably extremely small indeed.

If you’re arguing that the population who will want to sideload will grow after the deadline to encompass a large plurality

Sadly no. I'm with you there. Won't happen. And I'm also sure, that google's warn-notices will make sure to make it sound horribly bad like "You cannot install this app because it is not secure, the risk to your device is immeasurable, it might even shrink your penis!". Pessimism? Nah, would say realism. That's what always happen. Grab enough people with being open and great and cheap and caring, and when you secured a sizable portion of the market, cut the liberal shit out and control the thing to death. Normal enshittification of everything :-(

Hope yes, that's what rather puts us into the optimism-spectrum...But what do they say? An optimist is just someone who has not enough data :-)