Unfortunately, there are some false premises in this part of your post.
First, I don’t think anyone here is seriously suggesting that PureOS is a “private venture in OS development” or that this is the reason why it is a waste of resources. Those of us here who are arguing that PureOS is a waste of resources are doing so for other reasons than that. (To see our reasons, please read our posts above.)
As long as Ubuntu remains in existence and highly popular, this isn’t really a compelling argument not to use Trisquel.
Alternatively, if Ubuntu ceased to exist, Trisquel would likely just revert to using Debian directly as its upstream.
So, this part of your argument does not convince me that creating/maintaining PureOS is a better use of resources than just using Trisquel would be.
As you probably know, though, even though Debian does not ship any proprietary code by default, it is nevertheless not an FSF-endorsed distro and sadly it will not become one any time soon, despite the longstanding and laudable efforts of Stefano Zacchiroli et al.
Trisquel, by contrast, is already FSF-endorsed.
So, again, just using Trisquel directly would seem to be a much more efficient use of resources than developing/maintaining PureOS.
Trisquel is also a Debian-derived distro, for much the same reason. So, again, PureOS seems unnecessary.
Trisquel offers Gnome. There is nothing to stop Purism shipping Librems with Trisquel installed and pre-configured to use Gnome by default. This would by much less work than creating and maintaining an entire distro.
Is it? It seems to me that more is being lost, in terms of developer resources available for other, more urgent matters, than is being gained, given that Trisquel exists, already satisfies the FSF, and could be a drop-in replacement for PureOS.