Step out of the Walled Garden.
Other smart speakers have Spotify and play
Apple Music (and
airplay) just fine (and about
any other service too). Since this thread is a rumor about possibly getting YouTube music (which- as rumor- means it may not happen too),
other smart speakers already have it.
Or if airplay is "good enough" for all such purposes, get some "dumb" speakers which will play
anything linked to a Receiver that can airplay. Spotify (and YouTube, AppleMusic, Amazon, et all) will work great that way too.
Else, buy only Walled Garden products and live with the gatekeepers decisions within that wall. They decide what "their" speakers
you own(?) can and cannot play natively. They decide whether to allow strong competitors to natively play on "their" speakers and possibly have some impact on sales of their own music service. You own(?) it but these strangers decide what can be played natively on it.
IMO, speakers should have no gatekeepers: let any "smarts" live within the computing devices people use. Marrying smarts to speakers likely means the long-term is going to be like iMac where perfectly good "dumb" screens have to get tossed soon after the gatekeeper decides to obsolete the hardware and/or those "smarts" conk... even if the screen itself is still perfectly good and fully usable.
Dumb speakers can last- and sound just as good- 10, 20, even 30 years after purchase. I suspect the use window of any "smart" speakers is going to be much shorter than that... not because anything happens to the speaker tech itself... but because it is dependent on relatively short-term support for OS "smarts."
But if one wants smart speakers anyway, my opinion is to go with those that are minimally self-serving, already supporting pretty much
all music services and, ideally, with a hardware way to inject music into them should the parent company decide to obsolete/vintage the "brains" to spur on recurring revenue via replacement sales. With both options, the "owner" has a better chance at getting to enjoy the speaker portion for a much longer period of time... and enjoy it with
ANY source of audio vs. only those that direct play on (stream to) computing devices.