backbone routers' priority settings for ICMP & UDP

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

They work well in a situation like W. Richard Stevens supplies, such
as a local workstation shoveling packets into a SLIP link runs out of buffers.

The usefulness breaks down in a larger Internet with more serious congestion
problems. It could be the guy on the end of a PPP connection receiving
source quenches from a big router out somewhere.

Steven's qoutes "Although RFC 1009 [Braden and Postel 1987] requires
a router to generate source quenches when it runs out of buffers, the
New Router Requirements RFC [Almquist 1993] changes this and says that a router
must not originate source quench errors. The currently feeling is to
deprecate the source quench error, since it consumes network bandwidth and
is an ineffective and unfair fix for congestion."

Or is Source Quench really broken by design?

I wouldn't say that it's broken, it just isn't a desirable thing to use