Yes, they are replaceable.
I fried a bunch of cheap grinders and then bought a DeWalt for about $120. It was very powerful and I loved it. And it died. And I sourced the parts, which was easy to do, and the parts were very close to the cost of a new machine.
So i went back to frying cheap grinders. I bought a $30 7" grinder I use for hard jobs. It draws 15 amps and with a 4 1/2" wheel it is ferocious--much stronger than my DeWalt. I have a couple $20 4 1/2" things--I keep one with a cut-off wheel and one with a wire wheel.
That's the best grinding value in my opinion. A 7" cheapo for the hard grinding jobs and a pair of 4 1/2" cheapos for the cutting and cleaning. The DeWalt could do them all, but it's dead. Using human waves of Chinese grinders is more cost effective.