The last time I had a bike that used coaster brakes, I think I was about 10 years old.
I didn't worry about wear or maintenance, and didn't stick with a bike for longer than a couple of summers. I'm sure most kids are the same, so I figure it'd be pretty rare that anyone would be bringing a coaster brake in a shop for service.
I like the simplicity of the bike and utility of the gearing and brakes for wet weather (I've worn through rims on my wet weather, caliper braked bike resulting in a wheel collapsing after I applied the brakes).
I wouldn't know when those grooves would be worn off (unless I took apart the rear hub) but I guess I'll see how it goes. I do have the front brake that'll help in a failure situation.
I'll just keep going until the coaster doesn't work anymore.