This is all true, but if you specifically talk about mechanical diaphragm fuel pumps, I've never heard of any car fuel pumps having to be replaced every five years. Were a y of the diaphragm pumps you replaced less than five years old? Or were they 30 year old EH utes etc?
While it's true an aircraft ENGINE has a harder time of it than a car engine, I don't see that an aircraft FUEL PUMP is under that much more duress.
And if so, then ditch the diaphragm, and go with a piston design (which I imagine was the Billet Pump solution) . They had tested theirs to 2500 hours, which is 500 more than the engine is supposed to go for.
If non rubber diaphragm, you don't have the time constraint, just hours operated, like the rest of the engine.
It did sound like the Billet Pump fuel pump was more expensive than a standard Rotax one, but since it was supposed to last longer than the engine, you would no longer have to worry about it.
Look at it another way. You used to be able to rebuild fuel pumps. Not so the Rotax. Nice little earner for Rotax. Why sell you a $100 kit, when you can sell a whole pump?