I think that's pretty much beyond debate. Increased regulation, both for PRM and for safety standards, has led to a marked decline in rolling stock, I'd argue since the early 2010s, but for express services people may argue much earlier.
For an example, I suppose you could argue Hull Trains 802s offering power in standard whereas LNER don't, despite being far cheaper for the same journeys - likewise GC 180s having more comfortable seats though to be honest, due to the feeling they may be about to self-destruct, I'm not honestly sure I'd rate them a superior experience than Azumas.
More local to me, I suppose you could argue that on a hot summer day, the TfL stopping service on an air-conditioned 345 (if indeed, one is operating the service) is preferable to a non-air-conditioned 321. Seating wise, however, not so much