That is just untrue. Maybe where you live or during your lifespan but in many many areas, small barns used to be successful and they used to dot the landscape and be hidden inside of cities. That’s where many many people learned to ride, back in the day.
In the 90s I lived in a college town in a state that had a lot of rural areas. Despite that, there were at least 4-5 prominent facilities right in town (within a 10 minute drive) that did lessons. I think the biggest had 15 lesson horses. They were run as businesses, I don’t know where the assertion comes from that all small barns are subsidizing your personal horses but that was not the case 40 years ago.
They were driven further to the outskirts by development and now they are all an hour outside of town - those that have not closed or transformed into show barns with only 3 lesson horses and the rest owned or leased horses.
Most large operations around me in the area I live in now have 1-3 lesson horses and they quickly push people into ownership. Every boarding facility within an hour radius requires lessons and/or training with board - at least all the ones without barbed wire fences and terrible maintenance.
The landscape is changing and rapidly. The area I live in now used to be the king of saddlebred training (STL) but taxation changes to livestock rules and rapid development in the cities pushed that all to KY or further out to Columbia.
Dressage? Not one barn in our area has lesson horses. If you want to take dressage lessons you’d better be prepared to own and to pay a trainer.
The hunter scene is popping but those barns are all pretty far out and they push you into rated shows quickly. I know - I have one of their discarded lesson horses.
I think the western scene here is a little different but again, I don’t know of any barn with a huge lesson horse string that I’d say “yep, go there - they’ll teach you strong basics”. The ones who have the knowledge are running serious AQHA programs where again, the goal is ownership.
I wouldn’t touch the main huge facility with a 10’ pole. People I run into who have spent any time there have abominable horse handling skills, and horses have been injured and died due to terrible maintenance. The whole property floods.
Anyway - the smaller facilities used to be a way to make a living. Not to get rich - no one is saying you get rich doing it - nor even to have enough for vacations etc. But I had many very good trainers in my youth who lived exactly that lifestyle and did ok.