This is no longer true as OpenWRT now supports DFS
Strictly, yes.
But, almost by definition, a willingness to flash OpenWRT means a consumer has educated himself on some of the issues, has found the stock firmware to be wanting, has found an alternative firmware, and is both willing and technically able to flash OpenWRT and turn it into the router he wants.
Your point is right - that OpenWRT can overcome some of the vendor-imposed limitations, and is getting better too. I used it myself to
partly recover capability closer to the UK limits - but still not enough to cover the whole house. In my case, consumer-grade hardware plus enthusiast-grade firmware plus knowledgeable user still didn't cut the mustard.
But consumer-grade hardware, with stock firmware, in the hands of the average consumer, still seems to be badly limited.
[Aside:
In researching for this thread, I see that someone has tweaked a little more power out of the WDR3600 this year, on some channels, but still not enough. Too late for me, of course, as I've already moved to something where I don't need OpenWRT - on the very bleeding edge - to behave. The WDR3600 is now sitting in a cupboard, waiting for next time I need to fall back to the 4G modem.]