We were told that instead of getting books full of crunch that adventures would be released containing rule modules the fitted the adventure flavour , classes, subclasses, feats nad other mechanics. The example given was nautical adventure containing rulles for battle at sea, enhanced swimming rules, aquatic races, classes and feats. At the moment we are lucky to get a background. So this has nothing to do with what I wanted or expected but with what we were told we were going to get. As for your modular example Sacrosanct you can't honestly tell me we received the level of modularity we were led to believe we were going to get that the designers were trying to sell us on.
For example, in OotA they included a slew of DMs material with whatever new rules (crunch) was necessary. The thing is, with 5e you don't need as much actual crunch as in past games. That is, a lot of what's needed is to reflavor the existing rules, with some modifications.
Pages 17 to 30 is a mini DMG for adventuring in the Underdark and is exactly
the sort of thing they suggested. In addition, there are sidebars for each major area with additional rules, such as the Silken Paths section that details the general features for that locale, but includes rules for webs that are more extensive than those presented before. The Oozing Temple has rules for poor air and infant through adult Hook Horrors giving a general look at how you could do the same with other monsters. Basilisks get the same treatment later on. The appendices (pg 221-249) have backgrounds, magic items, monsters and the Demon Lords. Over 16% of the book.
SKT has 55 pages of Realms Lore for the Savage North plus another 15 or so pages in the appendix with more options, monsters, etc. More than 25% of the book.
PotA had 20-page campaign guide to the Dessarin Valley, 36 pages of monsters and magic items, a new character race, and 10 pages of new spells. Again, 25% of the book.
CoS had the 10 pages of Barovia (a mini-campaign guide), plus 20+ pages of appendices with backgrounds, monsters and magic items, or around 16% again.
Depending on the AP, they've had more or less, but they've had whatever new rules were needed for that environment or to enhance the story of that adventure. Those numbers account for far more than "lucky to get a background."
For me, they have had a great amount of useful material, even for a DM like me who probably will never run the AP. And I greatly prefer the approach to say, 3.5e books, which always had to have new races, new prestige classes, often feats, along with spells and magic items and sometimes monsters.
A lot of folks complained at the number of new archetypes in SCAG. Of the ones there, I liked about half of them. Some of my favorite material is the lore that ties the core races and classes closer to the campaign, but doesn't require a new archetype or sub race.
I love to have new magic items, spells, and monsters. But if the rules are well written and the game is well designed, you shouldn't have to have too many new rules in later supplements. Sure, some rules (you mention waterborne) can use some expansion, like they did in OotA for Underdark adventuring.
As for archetypes, feats, and races? My preference for those are in setting specific things. Like the Dark Sun campaign has a number of races that are designed specifically for that campaign. For casual gamers that just play the APs, new races and classes are fine, it lets them do something different for that AP, knowing that they'll roll up different characters for the next AP. For a long-term campaign like mine, you have players that want to play the new races, that suddenly appear, somehow fully integrated into the cultures of the world, despite the fact that they never existed in the campaign before. This was a big problem in 3.5e - oh yeah, they've been here for centuries and are common everywhere. You've just never seen them.
Most importantly, I don't think the designers are "trying to sell us on" anything. They didn't try to convince people to play 5e because of the cool new rules that will begin the APs, etc. They made a new version of the game and hoped you'd like it and play it. They said that they weren't planning on releasing a lot of splatbooks, instead they might incorporate new rules into APs as needed, but don't expect the flood of classes, feats, and such that 3.5e and 4e had.
The focus of the game has shifted (for the better in my opinion). In 3.5e and 4e, a certain amount of new crunch rules was required for each release. There had to be a new prestige class or archetype that was different than the others. I started trying to convert a number of those, and found that it was tough, not because of the new rules, but because they incorporated so much of what was already there, but streamlined them into a much smaller group - feats are a good example of that.
Now the focus is much more on the story again. That a good product has a good story, and good supporting materials to help the DM enable a good story. More mechanics usually aren't necessary for that. Making Barovia a gothic horror setting doesn't require mechanics, it requires atmosphere and supporting materials that help the DM in creating the feeling of dread and despair.
So I think it has everything to do with your, or others expectations. When they say that APs will include rules for a specific environment (such as waterborne or the Underdark), you are expecting that means something similar to older releases, with new class options, feats, crunchy rules, etc. I kind of expected that too, but when I saw they direction they've taken it makes perfect sense to me. And it does exactly what they said they were planning in my mind, although not necessarily what you, or others, thought they were saying. It's been a while since I've read the interviews or listened to the early podcasts, but I think they've been pretty consistent in following the path they've been laying out ahead of time.