I'm guessing the reason Apple didn't buy Sonos is because Sonos is technically not all that high-end anymore.

Don't get me wrong: I also use Sonos and I love it, but there are some quirks about it. For one, they are the victim of their own legacy. Sonos was one of the first to make streaming devices, which means they still use "old" hardware, or at least have to be compatible with their own older hardware (meaning users that bought a ZP120 15 years ago will still want to be able to connect it to their Sonos array). That's the reason they can't update their stuff with modern hardware, and older drawbacks of the Sonos system still remain and are not fixable because that would kill compatibility. Eg. Sonos still has about half a second delay when it plays audio. Not so much a problem when it's just audio or webradio, but it was a problem for their TV Playbar because you can't have voices delay when watching TV. They tackled that problem by having their Playbar and Play:3 range use a separate 5Ghz network instead of their 2.4 Sonosnet mesh network. It's a solution, but not a clean one.
IMO, this is also the reason why they have been flat out refusing to support Airplay or Chromecase natively for years, because then this shortcoming would become very apparent.
Another shortcoming of their lagacy is the fact Sonos only supports up to 65.000 tracks in their library. A lot of users have larger libraries than that nowadays. This limitation is also because of the hardware (too little memory) and cannot be easily remedied without making certain Sonos hardware incompatible with the Sonos system.

Sure, they are still the biggest and everybody knows them, but nowadays there are better alternatives, and people are starting to discover them. I'm thinking eg. about the excellent Denon Heos system.