Quite honestly, this is exactly what I am currently doing - identified a market with probably $50mm global TAM. Bootstrapping with first design partners currently.
One thing I didn't mention is that there are often a few sleepy legacy SaaS players (often public) in these niche markets who don't have the chops to add AI to their product and may be a good takeout / exit down the line. Won't be for billions, but if you bootstrap, that doesn't really matter.