I mean, this always seems to be the perennial problem with solo / casual players; until they can clearly articulate what they want, Blizzard is going to keep trying different solutions until something eventually sticks.
Or the game shuts down, whichever comes first. 
Define “rapidly”. 
It’s entirely possible that the target players for Delves are still struggling through them, and it’s the rest of us “tryhards” who are having an easy time of it.
On the other hand, if Blizzard tunes it to be difficult such that it takes time to progress, then they are accused of shoving sweaty e-sports down everyone’s throat. That, and / or, artificially time-gating content behind impossible mathematical gear checks.
How are they supposed to win here?
Is it, though?
One of the primary complaints is that Devles have allowed players to overgear for lower level M+ and as a result are starting at higher level M+ (where they can actually get upgrades) but are hitting a concrete wall due to a lack of knowledge / experience.
However, if they sign up for lower level M+ more appropriate to their knowledge / experience, then the complaint is they’re not getting gear upgrades.
So if Blizzard intended Delves to be a gear-up for M+, it seems to have backfired.
As for 2., there are many more activities than just achievement hunting for players who want to do more than raid log, so… 
With the way the Crests and gear breakpoints work, I can see why players might think this, but it’s also possible that Blizzard, never being one to do something simply when it can be done complicatedly, have opted for the latter, hence, the perception quoted.
I, for one, would just get rid of Valorstones and use Crests for upgrades, but have them match tiers (i.e. Gilded Crests upgrade Mythic, Runed Crests upgrade Heroic, and so on and so forth).
While it’s most likely a minority of M+ players, there is a sizable number who will continue to push past where rewards are given, for a number of reasons, so the idea players will continue to play beyond when rewards are given is not without precedent.
If Blizzard is staffed by devs with knowledge and experience, they are not taking their cues from Bellular. 
But isn’t this true of literally every streamer, nay, every player, out there?
Also, there are many streamers who are more solo / casual friendly, like Soul and Kelani. Kelani, especially, makes tons of videos on how to gear up as high as possible solo.
But you said so yourself, you do not know what the answer is.
We’ve had the discussion of “what do solo / casual players want in WoW”, and it’s still not clear what that is.
To me, it seems like what solo / casual players want is a system by which they can progress their character without increases in difficulty solo.
But that’s kind of sort of what Delves are, isn’t it?
(Note that I am ignoring the requests for player housing, since pretty much everyone will agree to that, and requests for “more quest content”, since it’s impossible to produce enough to keep players happy - they’ll always consume it faster than it can be created.)
And if even you yourself cannot say what you would want, is it reasonable to expect that Blizzard will figure it out, especially if most of their devs are not solo / casual players themselves? 
