The entire global military aparatas, from mobile headquarters, in-country bases, and submarines, is hardly a niche. And no, that's also not how it's done all over the world, that's how it's done in modern western countries. SMRs would work very well in rocky or rainforest regions, or areas where speed of deployment is more important (kick starting a region pre-solar, or brownout coverage, for instance).

Dont get me wrong, I'm making this point because SMRs have a place. I do believe solar should be the vast, vast majority of global energy production until something better comes along.