But if you just have a "congested bit" what happens is that you can "optimise" by half-implementing it, you just send as much as you can, and all your data gets "politely" delivered with an ignorable flag, while any poor schmuck sharing the link who listens to the flag throttles back more and more trying to "share" with a bully who wants everything for themselves.
This is a race to the bottom, so any congestion handling needs to be willing to get heavy handed and just drop packets on the floor so that such bullies get lousy performance and give up. "Essential" means it's a necessary component, not the first line of defence.
QoS for unfriendly networks has to work the same way. If you just have a bit which says "I'm important" all the bullies will set it, so you need QoS which offers real tradeoffs like "I'd rather be dropped than arrive late" or "I am willing to suffer high latency if I can get more throughput".