Even if it's based on Electron, their Linux desktop client at least exists and this is reason enough for me to keep being a subscriber. The web player is working but is missing features and has some major bugs (e.g., I cannot "dislike" a song on a playlist). My one wish would be for it to get on the same level of playlist management that Amarok, Foobar2000 and similar music players were on like 15 years ago...
Amazon Music's desktop client, despite also being Electron-based, is only available on Windows and Mac and its usability is so bad, I cannot even describe it. There is a reason why Amazon Music is a lot cheaper: it's worse in every metric beside costs that I were evaluating.
Electron does not make cross-compilation easy. It's about as cumbersome as building any other C++ project for multiple platforms.
That said, I'd very skeptical of investing in Spotify. Apple Music has a real chance of becoming the default service everyone subscribes to on their iPhone without thinking twice. Apple Music just have to be good enough.
I wouldn't worry about Apple too much, the market will become saturated in some countries after some years where they are popular. But in other countries, where they only use Android, who do you think will win the market?
Only in US.