You can't really "rotate" L1 keys without breaking HD playback on millions of old vulnerable devices and pissing off a ton of legitimate customers in the process. A hardware vulnerability can't be fixed with a software update. Instead, nowadays when a key is leaked usually Google just revokes the key of the unique device it was extracted from and not the entire model. But major streaming services often blacklist or limit playback quality on known compromised models.
As for L1 on desktop, Google did announce exactly that [1], but TPM would be likely too weak for that, so maybe something like Intel ME would be used. But so far there's no evidence of them actually rolling anything like that out. So for now if you want hardware DRM on desktop you have to use Edge or native Windows 10 apps with PlayReady DRM.
And about the APIs, streaming services do sometimes change up APIs, make old app versions stop working, etc., but often things work with no or minimal changes even years later. They do have to be careful not to break compatibility with all of their own apps across multiple platforms as well.
[1] https://web.archive.org/web/20210819093936/https://widevine....