Google will just make it a requirement in order to provide the Play Store and other Google apps on your device. Then most will be forced to follow along.

I could see it being the straw that finally breaks the camel's back for Samsung, though. They have their own assistant, web browser... they could just choose to go it alone.

This sounds like a cost to phone manufacturers, not a benefit.

At this point manufacturers don't have a lot of choices. To a reasonable approximation, the options are Android with google's blessing, android without play store, beg microsoft for windows mobile 10?, KaiOS, Tizen?, don't make phones.

About the only one that will sell the final product in the US is Android with Google's blessing.

Correct, Google has all the leverage to dictate what non-Apple smartphone manufacturers sell. Moving to Fuchsia would be expensive for everybody involved. Google is developing a new OS. Chip manufacturers will have to write new driver code. Cell manufacturers will have to integrate a new system. Everyone loses the work they put into the Android Open Source Project and their private branches of ASOP.

With all this cost, who's getting the upside? Google is clearly getting the lion's share.

If there's an ABI for drivers, then all phones can promise (more) OS updates with less work.

Treble is an ABI for drivers. OEMs have hardly changed their behavior.

Theoretically you could replace the AI layer (Topaz? IDR) with your own assistant (e.g., Samsung could integrate Bixby into the OS)

I should note that I have only glanced at what implementation details are public and it is entirely possible that this is not feasible