You're saying the devs would have needed time to playtest everything in master mode if they had created AI changes, which means they put out stat changes to save their time and not playtest the game.
You're misunderstanding what I said. Obviously playtesting would be needed for stat changes. But you need
more playtesting and developer effort for AI changes because there are more knobs to turn.
Global stat multipliers only really have a few knobs to turn: the specific multipliers for Hp, defense, damage, etc. So you play around with them and find a set of multipliers that makes the game do whatever it is you're interested in making it do.
With AI, you have a lot more experimentation available on a very granular level. Even if you restrict changes to just boss AI, you'd probably be going through 3-4 different iterations of AI patterns before you find something "good". Each AI pattern would require programmer time, not just playtesting time, since an AI routine is actual game-code.
That's a lot more time and effort than just tweaking some multipliers. And even that assumes that you don't need to create new sprites or other visual effects to signify these new patterns.
So why didn't they scrap the mode entirely?
I am not the developers, so I cannot tell you precisely what is going on in their heads (remember: I wouldn't have given Expert Mode stat changes at all). But they clearly think that Master Mode would be worthwhile to some percentage of their user-base. And they're right; just perhaps not as big of a percentage as they thought.
The devs should not have hyped master mode as much as they have, if it were only stat changes.
I admit that I do not peruse everywhere "the devs" interact with the community. But everything I've seen shows that the devs made it abundantly clear that Master Mode would only be stat changes.
So let's see if they'll add new AI for the game's enemies in the next updates (which should have few additions apparently) just to satisfy thirsty for difficulty in a game that is almost finished, which I think is unnecessary.
Master Mode itself, and Expert Mode itself for that matter, are "unnecessary". That doesn't mean they're fundamentally a bad idea. I don't see why anyone would wish for Master Mode to
not have more interesting AI.
I mean, you shouldn't
expect it and it would be wrong to be disappointed if they don't do it. But there's no reason to actively not want it to happen. It's not like new AI would make Master Mode
worse, now would it?