There’s also the LEO, security, and military markets (I would fit into the security market).
There are roughly 20 million desktop Linux users in the world. If Purism can get 1 out of a 1000 of those people, that would be 20,000. Probably 1% of smartphone buyers (1.5 billion per year) care about security and privacy. If Purism can get 1 out of 10,000 of those consumers, you have 150,000.
As the software gets better and Purism can demonstrate a working phone, the demand will keep growing. I don’t think Purism needs to worry about saturating demand in the market as long as it manages to ship something that has basic functionality and can last a whole day on a charge.
The Librem 5 is such a unique phone, that it will capture eyeballs, and I bet that many of the major tech sites will want to review it. I sat down and made a list of the 1050 innovations/milestones in mobile phone since 1973. I count 6 innovations in the Librem 5 that are new to mobile phones, which is makes it the most innovative phone on the market, since the Galaxy S III in 2013. I’m pretty sure that there will be a long term market for the Librem 5, because it offers features that no other model can match.