That is not how blizzard works here. Let me show you an example :
The look at data and reach a conclusion on their own. You don’t have access to that data, neither do the hard-core players.
Whatever you post here, is irrelevant. So what “hard core players” say is irrelevant to blizzard’s eyes.
No. They will balance tune things. The same way they do with Raids and M+.
Or do you think M+ will be all roses and rainbows with out bosses that 1-shot people, or mechanics that are absolutely don’t match the difficulty of the key ?
Well of course there will be balancing issues… ! 
Its impossible to do new features perfect from the very beginning.
The only difference with Delves is that with M+ they have decades of experience. Unlike with Delves. Be realistic here.
And I will finish this discussion by saying the following …
You confuse “hard core” as in “time spent in the game” with simple skill. You can play little, and play unorganized group content and still have skills.
And skill is something you learn, not something that falls from the sky. Problem is that you need challenging content to grow that skill, which until this expansion was exclusively organized group content.
Delves give you a chance to improve on that. And I hope you enjoy that journey and become a “hard core” player yourself (as you define it, not what it actually means).
So TLDR its not “hard core” players saying its easy. Its people with more skill than you that say that. And not all of them will go do M+. Some will, some wont.