Mouseferatu
Hero
To me, that's a demonic grunt, not a demonic captain. Contrast the d20 Marilith, that mirrors a high-level fighter in that it gets what we now would call extra attack, it sees through illusions and invisibility, it can even throw up a blade barrier for some battlefield control of its own.
I'm confused. The marilith in 5E gets seven attacks on its turn; how does it not have the "extra attack" equivalent? (Or more, if it chooses to use its reactions for opportunity attacks instead of parries.) And it has Truesight to 120 feet, so it does see through illusions and invisibility.
Seems to me that all it's lacking is the blade barrier, in comparison.
(I'm not trying to say you're wrong across the board. I'd like some added complexity in a few high-level monsters myself. But I'm not sure this is the best example.)