I don't disagree with you that in order to sell, these devices need to be somewhat appealing to more than just devs. However, I will say that the dev market isn't as small as it once was. A decent phone with an open platform would be something a lot of devs would likely prioritize buying. It won't be the next Iphone, but it will be a pretty dedicated market segment.
Framework is a good example of that. A laptop business that stays afloat mostly because there is a desire for repairable long lasting products, even if it's a bit niche.
Given a lot of phone manufacturers are now trying bizarre edges to get ahead (like foldable... who wants that?) it seems like a good rarely taken route.
