Will the controller use the new AppleMusic API?

I seriously doubt it. In the good old days Sonos wrote specific code for Rhapsody and then Pandora services. Expanding to more services meant they were on a hiding to nothing, so they invented SMAPI, and said to everyone who wanted to participate: hey, use this. And they did (~55 or so to date). Eventually even Rhapsody used SMAPI, and Pandora is the only hold-out today. This meant that Sonos can support zillions of music services without having to write to zillions of different APIs, just two (SMAPI and Pandora). (The Pandora API is simple, and the Rhapsody API was the worst API you could ever imagine: good riddance to it).

For Apple music support, Apple had to stick a SMAPI interface over their service, which they (eventually) did. If they wanted it to look more like whatever the native iOS app does, then I'm sure Apple could do it, if they wanted to. But Apple are not in the habit of opening their stuff to non-Apple clients, and while the original SMAPI implementation was an exception to Apple's traditional f-u-to-everyone-else policy, it seems they are not interested in updating it. Plus now they are producing a Sonos-compete product themselves, so why would they.

I would expect Sonos to continue to use SMAPI for all services (except Pandora), though it needs some improvements for easier Alexa support. I would seriously doubt they would use the AppleMusic API, assuming that API is even available on Linux (which is what the Players run).

Thanks for your answer. Unfortunately that means I will stop investing in Sonos and search for something else as currently AppleMusic is not usable with the Controller.

By choosing Apple Music as your streaming service, you are accepting to some extent that you will be locked into the Apple ecosystem. With the advent of HomePod, that lock-in will increase. That is The Apple Way.

I'm substantially bought in to the Apple ecosystem, too, but chose Spotify over Apple Music to mitigate the lock-in effects for music.