I don’t think market saturation is really a thing to worry about. Only if you narrow it down to the nerd niche (although still many more nerds per year are born than Librems are produced).
But that is not their (only) target demographic. I guess it is best summarized as people who want privacy and security and oppose (consciously or subconsciously) planned obsolescence and surveillance capitalism.
Besides the nerds, Purism already has security and freedom minded businesses as customers for their laptops and servers. It has also been speculated here that security minded companies, organizations and agencies might be very interested in the Librem 5 (USA).
They also want to appeal to “ordinary” people, and depending on their specific needs and how much they are held captive by unfree technology, this can already work to some extent.
The Librem 5 is undoubtedly the most ambitious project of Purism so far, but I can’t see how the team might ever need to shrink (below its current size) for the foreseeable future. Several additional product lines are already envisioned (tablets, NAS/router) or wished for by customers (desktop PCs, printers that don’t track you).
So, I guess for the next decade or two they will not feel the need to think about toasters 