I'm not sure it's demand so much as extremely high barriers to entry. Android and iOS are both mature and have a lot of features, plus manufacturers like Apple and Samsung have a lot invested into the latest and greatest SoCs and other hardware features. Matching all that right out of the gate and building competitive hardware and an OS is hard, making it difficult to make something that the average consumer will notice.
Tbh, I have more hope in the Pinephone because they aren't trying to be too pure in that they're supporting Android to start but hopefully will allow other OS'es as well.
Doing the whole nine yards (in addition to trying to make the thing blob free) is going to take more than up front investment that other, already established manufacturers don't have to deal with, which is giving them a handicap in a fight they start behind in.
