As a point of comparison, Microsoft ships its OS across a variety of manufacturers and largely keeps it maintained across them (give or take some exceptions like enterprise environments & the like).
Even unlocked Android phones purchased independently of carriers have inconsistent lengths of support, so it doesn’t seem to be entirely a result of carriers, so…What happened here?
My 10 year old Thinkpad barely qualifies as “running” windows 10, not Ubuntu for that matter. Haven’t bothered trying 11. I do partly agree with you, especially moving forward. But an iPad mini 2 has 1 gb of ram and 16 gb of space, both rather huge limitations for a mobile OS of today.