For an expedition bike like the Nomad, I like to have a healthy amount of clearance between my 57mm tires and fenders in case I get some mud on my tire, etc. First photo shows that I have a small spacer between my fender and frame for my Nomad rear fender, second photo shows I have almost no gap between the front fork and fender. Maybe you could put a 650b tire on there if you wanted skinny tires, but skinny tires is not what the Nomad was designed for. I think it is pretty much a 26 inch bike.
I think lack of tire availability was a much bigger problem before the internet. I have an old bike that I quit riding in the 1980s because I could no longer find tires to fit it. They were out there, but not where I was looking. The bike took 650b tires, which have become common again and been re-branded as 27.5 to give them a new modern sound. But now with the internet and huge number of suppliers that sell over the internet, I think worries about lack of 26 inch tires in a few decades is not a big concern.