Pretty much, why can't I run a modern kernel on the average device running Linux, whether that be an IP Camera, or a smartphone? Why are we forced to run ancient, unpatched & insecure software by Qualcomm, LG, Samsung & Google?
Qualcomm is much more iffy, there is some work on integrating support to mainline but it's only beginning.
You lose the warranty bit and still cannot replace FCC-approved modem firmware, because certification reasons.