backbone routers' priority settings for ICMP & UDP

Since source quenches are not supposed
to be used on routers anymore, the expectation of receiving a source
quench on a large network (like the Internet) is a bad one, so the TCP
implementations have to implement congestion controls through other means

As is (reasonably) well known, TCP has its own congestion control built
in to an extent. However, if your network is UDP heavy (for instance)
on a protocol which has no higher level congestion control, why are
source quenches from routers worse than nothing? If they aren't, then
wouldn't ignoring source quench on the client for TCP have been a better
strategy? I'm thinking about this in a WAN context where theoretically
you have more control over the clients as well as an Internet context.
Or is Source Quench really broken by design?