What I find most interesting is, apart from monsters being even more popular, many folks seem to agree with you. This has some implications for WotC's design goals...because it means the invested fans may not be as durably on board with the "less is more*" philosophy as either WotC or the fans themselves thought.
Subclasses were meant to break class free of rigid structures, allowing effective support of previously multiclass-only concepts without needing such rules, and being really diverse, adding much needed richness and depth. Instead, they are all too often held back because changing the base class too much isn't permitted. Further, because a small minority (~35%) can block any new developments if they're vocal enough, concepts that could have partially pushed things back toward the intended goal (such as subclasses available to multiple classes) have been abandoned for not being resoundingly popular.
Likewise, the strong "magic items are optional" stance was meant to free groups from the unpleasant feeling that such items were mandatory, making them a fun and flavorful opt-in choice. Instead, I've found magic items to now be decidedly excluded in many contexts, with many DMs reluctant to ever give out anything more interesting or valuable than a handful of +1 weapons. Far from making magic items magical again, it's mostly just made magic items unattainable, weakening the feel of fantasy for many tables.
As for the lack of new rules, honestly I just blame the death of "modularity." 5e really could have been a modular system at least somewhat like what they totally-didn't-promise-but-really-wanted-folks-to-feel-was-promised. It just would have required actually serious, rigorous playtesting, coupled with specific and clear design goals and an aggressive development schedule. Instead, what we got was very lightly playtested, decidedly un-rigorously, with vague and often useless design goals, and lots and lots of dithering about. (I mean, it took them nearly two years of playtesting to settle on just the Fighter!) 5e wanted to offer rules diversity but balked at the effort required to make it work, so we got what little we got and minimal effort to grow beyond that.
*Except spells. With spells, more is always more. Most books that contain any player content at all contain spells. Bluh.