Because the main apps used on other systems work on Linux, and also games.

Linux phones now is more Linux in early 2000, a few apps that kinda work, you don’t have banking apps, games, android auto like, streaming apps or any of most common apps used by users

“Linux phones will skyrocketed” by who? Nerds, the comum user doesn’t care if Android is proprietary or opensource, they don’t even know.

There’s already other mobile OS but have the same issue, no apps or developers interested on making apps for it.

FirefoxOS, Ubuntu touch, something else Linux based, it’s for some nerds play on a secondary phone.

Comum people don’t stop using Windows or macOS because it’s proprietary or a privacy nightmare. EU didn’t stop selling iPhones because it’s proprietary.

It’s the device vendors that decide which OS would ship, and people will use it

Do One Thing Well: Each program should focus on a single task and perform it effectively.

At the moment im not hosting a music server, but used to use Navidrome, it worked fine and used a small footprint.

Having all in one it’s more issues to solve, if something breaks, everything breaks.
Having all on Jellyfin is more convenient.

But adding hundreds or thousands of songs along with movies and episodes will create a huge database, more resources used, slower searches

You can have GRUB menu on boot with UEFI.
Only shows black screen if you disable the boot menu, some distros hide the menu by default, even with BIOS.
https://wiki.archlinux.org/title/GRUB/Tips_and_tricks#Hidden_menu

But that one clearly shows BIOS