I think good PWA support and Android minus Play Services support could actually cover a lot of the bases. Both of those technology stacks are FOSS; PWA's alread

I think good PWA support and Android minus Play Services support could actually cover a lot of the bases. Both of those technology stacks are FOSS; PWA's already cover services like Twitter and Airbnb, and are likely to become commonplace now that Safari intends to implement Service Workers; F-Droid has a huge back-catalogue for open source Android Apps, and a lot of important closed source Android apps don't rely on Google's proprietary API's, for instance Whatsapp, or Moovit, or indeed the random parking app used by my local town. But it seems to me the various open smartphone projects are not keen to support apps beyond the native Linux toolkits. I really think that attempting to replicate the basic functionality of the Android or iOS ecosystems entirely in Qt or GTK is forlorn. The platform will start with very few users (the Purism smartphone would have 2500 users if it meets its funding target), so companies are not going to build to it. It would rely on community efforts, but that's a huge amount of effort, and even then assumes that the API's are available. Having said that, I'm thinking of backing LibreM, as you say the hardware is very important, and to be applauded. I mostly just want good smartphone hardware which runs on the mainline kernel. Software support (for vanilla Android, Lineage OS, Sailfish or Plasma Mobile) should then follow from that by default.