Linux barely exists as a desktop operating system as is. The only difference nowadays is smartphones don't have that freedom of customization desktops provide. Smartphones are consumer centric devices, you can't even swap batteries anymore. Leading OEMs have no interest in supporting an obscure operating system, they know the majority of users want Android.
When Purism releases this as an OOTB product, it will without a doubt run on some no named Chinese Phone that holds the mediocre power of any low end Android Device. Only it will be $600, rather than $180
When it gets stagnant, companies will stop innovating that hard and slowly but steadily open solutions will gain quality and feature parity.
If in 10 years I get an open phone with the features close to what I have today, I'd be pretty happy. Because I just don't think phones today are going to be that different in 10 years.
You are right that Linux Desktop barely has any market share. But it's a very viable solution and I'd personally argue that it's ahead of it's competition.