I've got your back on this one. For what it's worth.
First, I think the current - dating back to the late 1930s, from what I understand - AASHTO guidelines on U.S. route numberings should be followed. Their own guidelines, or policies (I dare not say "rules") say that short single-state routes and routes with directional suffixes should be eliminated or renumbered to conform with the guidelines.
Second, I would say that AASHTO and its predecessor organization hasn't followed its own guidelines. As an example: it approved a very short U.S. 156 in Kansas in the 1950s, and all the 4xx routes that don't follow any numbering pattern. (Did I forget 163?).
Third, look at the definition of a U.S. route. Nothing in their definition would support having a U.S. route act only as a city route or emergency detour for an interstate route. If a route is not the "shortest and best [route] between major control points on the system", it is probably not consistent with AASHTO policies to maintain a U.S. numbering when another route meets that definition better.
Finally, I think a given route should have as few identities as possible. As an example: when I drive from Pueblo to El Paso, I take I-25 and I-10 because those best identify the highways constructed within the interstate highway system. Having this route also designated as U.S. 85 does nothing to enhance navigation. Its former routing has no independent identity (serves no major population centers) anywhere within that 600 mile length.
There is a philosophical divide of sorts between routes constructed in the east and the west as far as whether the interstate was built on top of or supplemental to the previous U.S. route. I-25 is a prime example of a route built on top of the previous route, and former U.S. 85 only exists as short, disconnected routes through major cities along the interstate. The former routing in southern New Mexico (current NM-185 and 187) only serves as a local arterial connecting small farming communities. Lots of places along I-25 have no significant alternate at all.
In the east, the interstates were in large part constructed adjacent to the former U.S. route, but those roads continue to serve mainly local traffic. It's a totally different thread to argue whether, for instance, a U.S. 11 should continue to be identified from Louisiana to New York when nearly all that length has a nearby interstate.
So, of the decommissioned routes lamented in this thread, I only think U.S. 89 and, if you want to include its twisty alternate 89A, was an unnecessary decommissioning, at least as far south as Wickenburg. I think Minnesota ultimately intends to turn back to county jurisdiction U.S. 61 from St. Paul to Wyoming (closely paralleling I-35E and I-35, and mainly carrying local traffic), so that leaves a concurrency of I-35 and U.S. 61 for about 150 miles. If you look at the routing, U.S. 61 approaches St. Paul from the southeast while MN-61 approaches Duluth from the northwest. There are shorter routes between Duluth and, to name a significant route along 61, La Crosse than going through St. Paul. Thus, route continuity is probably not important.
Oh, and 210 and 371 were single-state U.S. routes.
Gasoline, meet match...