I'm not buying this whole propriety thing. No pun intended.
I'll use KWA as an example because that's what I have.
When you hear "proprietary parts" don't assume they're hard to find because they're not. I did a search and found both a KWA gearbox and a King Arms gearbox in approx. the same amount of time - One on amazon and one from the proshop at KWA.
Parts from the KWA proshop are pretty much the same price as other brands found on amazon or within a reasonable range. I did not compare all components but things like grips, hop ups and gearboxes were no cause for concern - a max of 15$ more for the gearbox.
As a KWA owner I'm not worried about not being able to fix or upgrade my rifle, and while I would say the proprietary issue is something to "consider" - it's good to be an informed buyer - it's not nearly as much of an issue as it may appear.
Check for yourself. Hypothetically try and buy every part. Compare prices, ponder....profit??
Here we are again bro -just you and me. Same kind of moon same kind of jungle. Real number 10 remember? Whole platoon, 32 men chopped into meat... We walk out just you and me, nobody else. Right on top huh? Not a scratch... Not a fuckin' scratch... - Sgt.Mac, Predator