Mobile is not all that counts. I don't know anyone without at least one laptop or desktop in the house (typically more) and gamers alone contribute more billions of $$ to entertainment than sports and movies combined. There's hundreds of millions of normal people who are still using MicroSlop Windows which has turned into a surveilance nightmare almost as bad as mobile platforms simply because it's the lowest effort/barrier to entry (pre-installed).
I absolutely want to concentrate energy on FOSS on mobile - with you there, but Steam Proton has made gaming so simple on Linux and the last 10 years of quality of life OS/Kernel improvements means FOSS can already compete on "desktop" and win. I"m saying don't dismiss it because if you can prove to people they can have a bulletproof and seemless experience for FREE without having to pay subscriptions and get privacy in the deal, they are more apt to consider a Linux phone (assuming it works).
P.S. I should also mention that most everything we improve or build for desktop Linux can be used on mobile (within mobile plat limitations). Win hearts and minds where you can - Linux isn't just for servers.