Is there a rule saying "You have to be this fast to buy carbon"? Nope.
You used to have to choose between tubular disc or clincher+cover, but it's been a few years that the Zipp Super9 and FLO Disc have been out. Along with the HED Jet, that's three price-points of clincher disc wheels that offer great performance. People are more educated about aerodynamics these days, and are more likely to ride a disc full time rather than the "Maybe it's too windy/hilly for a disc" myth that some old-timers still repeat. If you're going to ride it full time, at every race except for (hopefully) Kona, the cost isn't quite as scary anymore.
Plus, outside the US it can be easier to get a disc than a cover... and covers are still more of a faff. I still have my cover at home (my partner rides it) but it's fragile, can slide (yes, I've taped it) and I don't buy the argument that it's as good as a completely smooth disc. WheelBuilder.com are welcome to convince me otherwise, I'll split the tunnel bill 50-50.
ZONE3 - We Last Longer