That's certainly common!
I think ASICS' running shoes were a fun example of where this was probably not the case.
- I certainly didn't notice a difference in that time, though I'm admittedly not much of an actual runner
- Serious runners might notice small technical differences, but the negative user reviews didn't seem to indicate those were the people making the complaints
- The overall user reviews remained positive
- The competition in the shoe market is incredibly fierce; I'm not sure a brand could tank their quality and survive for long
- Let's not forget the other big variable: the wearers' bodies, particularly their feet. Now, those definitely do change over time -- most often for the worse, sadly!
- I doubt anybody was blind A/Bing a pair of Nimbus 20 against a pair of Nimbus 21 or 22 or 23 or 24. At best, a longtime Nimbus buyer is probably comparing a brand new pair of e.g. Nimbus 24 against their degraded but broken-in Nimbus 23 and their memory of how the Nimbus 23 felt when new. (And their body is a year or two older at that point..)
- Because it's such a long-running line of shoes, there could certainly be year-to-year variations... but it's hard to imagine there was an actual 5 or 10 or 25 year downward slope. I mean, otherwise at that point the shoes would just be instantly injuring you or falling apart in a week
- Also because it's such a long-running product line and (aside from bleeding-edge professional marathon/track shoes) sneaker manufacturing in general kind of seems like a solved problem... it seems like all of the possible cost optimizations have already been optimized. I don't really think there's much of a manufacturing or bill-of-materials cost difference between $20 running shoes and $200 running shoes anyway -- I'd be pretty surprised if "quality shrinkflation" was really much of a viable way for ASICS to save a few pennies.