They’re gonna break iMessage for the ~50% of population not on the latest version of iOS next week?

...They obviously would do it in a way that doesn't do that? Do you think this company can outsmart them in the long-run?

This is a replay of the Messenger Wars. Yahoo IM, AIM, ICQ, etc played a game of cat and mouse with clients that wanted to get all of the messengers in one spot.

Right. What is stopping Apple from requiring a valid unique device key with every request now?

Old hardware that doesn't have it. Are they gonna ban my iMac from 2009 that was before iMessage was a thing?

Yes, they are. You already don't get OS updates.

No they aren't. My iMessage still works, and so will other devices that had iMessage.

just because it works now doesn't mean it will tomorrow. You're on officially unsupported hardware / os version.

Ask someone with 32-bit Mac apps about Apple’s willingness to leave parts of their userbase behind.

They still work. Apple doesn't stop services from working or programs from functioning.