My phone is my phone, not Google’s. They have absolutely no right to prevent me from running whatever software I wish on that phone.
This must not be allowed to stand.
You didn't fund the development of the OS, contribute to it (presumably), you didn't market it or position it alongside your brand.
I'd agree with you if you said you have a right to run anything on the hardware under a different OS, but you have no god given right to run whatever you want on the OS.