Maybe something has changed on the planet that I don't know about.
Yep, a lot did change.
Maybe the newer receivers are so good there is no problem
That's what changed. That, and better, more narrow transmitters as well. There is a WORLD of difference between the gear available when those articles on the AMA site were written and the quality of even a cheap 4-channel box today.
PCM is nice, and might help, or it might not. I wouldn't say you are imune to problems just because you have PCM, that's for sure.
However, there are lots of things going on where many TX's are running, sometimes close together, with no problems. Sounds like the glider guys might actually be harder on their RX's then we are in combat, they sure fly a LOT farther away at times.
Anyway, in the case that started this thread, I'd only believe 3IM if there was some other faulty equipment involved, like an old RX, a very weak TX getting interferred with by a couple of overly strong ones, something like that.
But if you want to limit the number of planes up, power to you. It's not necessary, and since you're not at my club, it won't bother me.