The aftermarket vs OEM debate for me comes down to this: Other than a bunch of saw enthusiasts, most people are not going to invest the money required to "restore" a saw to an OEM condition. Even for pro saws, the majority of them are under $1000 and if a pro gets xxx hours of use out of it, they're going to get another one and carry on.
Aftermarket parts allow the resurrection of saws that would stay dead if there weren't economical options. While most would prefer a saw to be repaired with OEM parts, few would be willing to pay for that status. I rebuild a lot of clamshell saws because frankly, they're not worth fixing. With cheap parts, I can refurb these and still sell them at a "profit" (as long as my time has no value, lol)
So in my opinion, the aftermarket parts aren't taking that much market share from the OEM manufacturers because most aren't going to buy the OEM parts anyway. What the aftermarket parts do is keep name brand saws in circulation that would otherwise be in a dumpster or in a pile in the corner of a shed.
Someone in this thread mentioned a Husky 55 cylinder kit came down from $200 to $100. To me, it's "so what". These saws sell for $175.00 used. Why would someone spend $100 or $200 fixing one? I know, I know, because we're all nuts. But the "real world" just isn't going to do that. Anyway, that's my 8 cents.