I see the complaining over Route X more to do with the track's uselessness relative to the offerings in GT Sport specifically. GT Sport is focused towards structured competitive online racing and realistically, and the track doesn't offer any true racing potential (except through certain one-make matches, and drag racing, but the latter is yet to have a sanctioned mode in the series for well over 15 years). Ultimately the winning car is pretty much* decided by having the highest top speed, either through power or optimal gearing.
The comparison to Avus isn't quite 1:1, not only was that a completely different era, it was also a public-ish road with a comparatively uneven surface, and the cars of the day didn't really "top out" in the same way they would now. Plus, even in it's fastest form, I believe it still had some points that required braking, whereas Route X's corners will hardly even yield advantages regardless of how much banking you use. While Avus is a more "basic" ribbon than most other circuits, it had far more surface variety than Route X.
Route X functions well as a test course, but only up to a certain point - it's basically just a straight line (yes, I know - there's two corners - but do any cars in the game even need to slow down to get around them?), and can really only provide a good base for optimizing transmission setups. That's useful, but that was kind of already easy enough to do on tracks available in the game, and it doesn't provide any new areas for testing suspension or other specific handling scenarios. If it did have an infield section and it came with a mode that offered such analytical software, then we'd be having that conversation - but it didn't. You could argue that it offered the best base for straight line testing (due to the precise distance markings and perfectly level terrain), but then the conversation begins to switch the pros of this marginal improvement/advantage vs. cons of not having a different track with completely different functions.
Route X was a track initially introduced during GT5's active phase. Assuming PD was able to use the assets and touch up, upgrade, and/or add to them enough to be a GT Sport-ready original course, then I feel it could be inferred that there are/were many other tracks of similar development status that could have been picked from. Here's a list, gathered from a brief scan of wiki and some of my memory:
Autumn Ring (Updated for GT6's launch)
Matterhorn (GT6 launch track)
GT Arena (GT6 launch track)
Apricot Hill (GT6 launch track)
Midfield (GT6 post-launch track)
Sierra* (GT6 post-launch track)
*Sierra was a course maker location so it may not have actually been a usable asset.
*May have also been possible, depending on the dynamics of real-vs.-fictional circuit development.
Twin Ring Motegi (GT5 DLC #3)
Ascari (GT6 launch track)
Silverstone (GT6 launch track)
Goodwood (GT6 launch track)
Daytona (Updated for GT6's launch[ToD])
Spa (oh, the memes) (Updated for GT6's launch[ToD])
Nearly all of these would have been able to individually serve more purpose in the context of GT Sport, exceptions being Goodwood and perhaps GT Arena. While Route X has a purpose, it isn't a very useful or expansive purpose, and has a very limited scope in comparison to virtually any other circuit in the real or digital motorsports world.
Ultimately the complaining (in my view) largely comes down to the fact that PD (***likely***) had a collection of 5-12 tracks to pick from, and they not only picked arguably one of the top three worst/most useless ones, but it also does have genuinely useless qualities. If Route X was literally the last well modeled* track yet to return to the franchise, and it was used as filler between their next new original or fresh-scanned real-world location, maybe there'd've been a different reaction, but that doesn't seem to be what happened. So here we are.
*I guess "well modeled" could be up to debate, tracks like Deep Forest, Trial Mountain, Grand Valley, and Coté de Azure seemed to have had very minimal (if any) changes from their PS2 counterparts in GT5/6, and a lot of the city tracks don't seem to have aged well.
