GrapheneOS is already a viable de-Googled and significantly hardened and improved fork of AOSP. It runs on Google Pixels at present, with an OEM device planned for release in 2027.

Hardware. Mass manufacturing, plus the deep pockets of a corporation, mean that we've come to expect cheap prices for inanely powerful hardware. Yes I'm calling an $1,800 iphone cheap for what you get. That's cheap for what you get because if you're a tiny company, you can't get a phone of that level manufactured that you can still for anywhere near that price, and that's a super high end model. How many people are going to shell out $1,000 for a model with the specs of a $500 model just because it runs Linux? And that's before you even actually deal with the software. Specifically, driver support, battery life, and app support are the three big show stoppers there. The best option this second is a Pixel running GrapheneOS, and that's based on Android on Goolge hardware. (They did just announce getting off Pixels tho.)

A Linux smartphone has been tried before. That's not too say someone shouldn't try again, but just to say there are lessons to be learned from those attempts.