I think there's some truth to what you say, in this general sense: Obviously, an amplified PA system recorded at a distance does not have much in the way of detail or frequency response (except in the low-bass). So, obviously, a high-end condenser microphone cannot pick up more than what is there. But, as others have already pointed out, there's still the matter of the overall accuracy of the microphone at recording whatever IS there, and I think the preference for certain high-end condenser microphones around here goes beyond just some kind of snobbishness about what they've cost. I've owned and used most of the brands, and I think that the person who originally recommended Schoeps to me was right -- they're "good at recording PA systems." Is there less difference between an amplified rock show recorded with Schoeps vs. AT 853s compared to a symphony orchestra recorded with those two brands? Maybe/probably. But the difference is still there.
I personally think the more steep diminishing returns are experienced in the preamps and A/D stages. First of all, I'm not convinced that the A/D of any prosumer or better machine is really improved upon much by an outboard. Even with preamps, I know we all have preferences for "flavors" of sound, but I think you're really entering speculative territory there. You can run a comp and tell a difference, but it's not clear to me that the "difference" is significant enough to warrant the use of that equipment.
Conversely, it's not hard to tell the difference in a Schoeps recording, a Church Audio recording, and an internal mics recording. That doesn't mean everyone will always prefer the most expensive one (especially without EQ), but the differences in the recordings are very, very obvious.