It's not a niche, in the same way that the Arduino isn't a niche. Back in the day the BBC Micro B had a thing called the "User Port". It was a TTL level bunch of pins you could attach all sorts of experiments to, controlled by a 6522 VIA. It was amazing, very few home computers had this feature (though the ZX81, Spectrum etc eventually got these add-on boards over time)
I for one, when writing BBS software, hacked together a thing to allow my BBC's User Port to detect carrier select, ring indicate and something else off of my old (pre-Hayes) modem to provide these signals from the modem's RS232 interface to detect calls and dropped calls etc, because the beeb only had RS432 which made it impossible to do this with (all you had was CTS, RTS, RX, TX and Ground).
The Beeb had an analogue port. You could hang all sorts of experiments off of that...external thermometers and the like. PC's (Apple included) "ruined" all that, and what external interface boards you could get to replicate this functionality were usually pretty expensive, because the world moved on, and this kinda thing wasn't built into the design spec.
The Pi was intended to recreate that 70's/80's fun, joy and excitement, and re-introduce the ability to do stuff like this that modern PC hardware simply doesn't provide for out of the box.
I don't mean this as a slight, but perhaps you missed out on that era, but have also missed out on the point of this stuff for "hacking" out ideas because where you've got an itch you need to scratch....like building a weather station out of bits.
The Pi, it's marketing etc is an amazing thing and should, and does, get folks back to hardware and software hacking for fun and intellectual curiosity without spending megabucks.
Sorry I missed the last part:
> "I want a microcontroller, but don't want to program a microcontroller."
But it encourages you to learn how to program a microcontroller, that's the whole point. Curiosity which leads to other things.
I had to fix things on my car but didn't wan't to learn or wasn't interested about how to fix things on my car...turns out I am now. And saved money. And learned things. And saved money. And satisfied some mysteries about how cars work. Again, to encourage curiosity which leads to other things.