You cannot install whatever operating system you want on your tablet, phone, or, soon, any mac desktop/laptop at all. The enthusiast/gamer market isn't enough to keep the intel/adm desktop market going. Soon the only place you'll be able to run a free operating system is on a rented vm in the cloud.
Not on Apple devices, but custom operating systems are alive and well on most non-Apple laptops, tablets, phones, servers, or even embedded systems with no signs of it changing. PostmarketOS is a great example.
