Also, I'm not sure the draw of a cosmetics only shop is that compelling. I haven't felt the need to buy a skin that doesn't do anything to gameplay.
Which the company is and has been spending significant dev time on.
For a criticism like "everyone is busy working on X instead of good things", X doesn't have to be in the same game.
It's entirely possible for example that they had plenty of animators and but ran out of time. Or they had animators and time but found that they had to compromise on some animations because there was too much going on on-screen and otherwise the frames would drop or the minimum spec would get uncomfortably high or whatever. There are any number of reasons that could be the cause here and we on teh outside have absolutely no way of knowing.
I hate what they did with diablo immortal as much as anyone but we don't know enough to say how that affected diablo IV if at all.