First of all, “Based on what who would expect?” is incoherent.
Based on what? Based on who? Expecting what? Why is it a question? ← What does this mean?
Secondly, you then start talking about me speaking for Blizzard? No? The observations I have made is stuff we know about in multiple different areas of the game (including other games as well). Putting a default barrier at one particular point creates an expected norm.
You put that norm too low, everyone rushes past it.
You put it too high, and it becomes a slog to get to.
This is why the norm is somewhere around an average and that average will dictate who your main audience is. For M+, there’s an expectation that people who want to participate in it will have the possibility to progress their character to doing 10s to fill up the vault with the highest possible reward that’s available from M+ for the purposes of progressing your character, even if the progression doesn’t stop there.
I think you are talking about people’s expectations and goals to push towards, but this why I’m saying that the average norm can’t be at the end of your expected character progression. Meaning that you can’t create systems that necessitate people to be at the end of the progression system for your character, before you can even begin to progress. Which is what ends up happening if one puts it at a +10.
For this we can look at the age old “given the opportunity, players will optimize the fun out of a game.”
That article is from 2011, and WoW already had a version of this before that since Wrath of the Lich King called Vault of Archavon, where a common requirement for people who wanted to get gear to progress their character was to have a higher gearscore than from the gear that was available from VoA.
In Legion, we saw what Titanforging did when folks were mass spamming Maw of Souls where people ran it 100-200 times at a quite low level but insanely quickly, just to hope for a mega titanforged item. Due to the effectiveness of it, once you were done progressing your character and you had no other reliable way of doing it, players started doing it the unreliable way … which was spamming Maw of Souls, finishing it under 2 minutes, and hoping for titanforged items.
When the expected norm of the game to progress your character exists in the game in these states, players do it every single time. You make resilient keys appear at a +10, then that’s the only thing that exists - nothing else. Which brings one back to the point I was making of “Why a +12” and my answer is a very simple one that I don’t think you are getting; I’m not tying it to an achievement, because it doesn’t need to be. It is just an expectation of “Can you reliably and regularly time +10s and showcase that you can do it at a rate that proves you wouldn’t either have, create, or be in the problem group of players that ‘+10 norm’ would create? Then, eh, sure.”
At a +12 range, you are pushing keys. You don’t have to push to the highest possible extent, you just have to showcase that you aren’t in the group of people who would en masse ‘optimize the fun out of the game’ - because once you are past that point, resilient keys wouldn’t be the norm, but an actual genuine quality of life improvement.
You are tying it to achievements and rewards, when it isn’t. Resilient keys are a quality of life improvement for some, but for others it would be Gearscore from VoA and Maw of Souls spamming from Legion. The entire point, no matter how one chooses to describe it or provide whatever reasons, is to facilitate the QoL-aspects and avoid the “optimize away the fun”-aspect of said mechanic.