@dave2008 - I think we're pretty much on the same page here. I don't recall anyone ever complaining that 4e monsters were too complex to run. And, frankly, I'm struggling to think of an example that even could be considered too complex. I opened my 4e Monster Manual and picked a couple of random examples - Aboleth Overseer Level 18 Elite Controller (Leader) - about as complex a 4e monster as you get. 5 abilities, all completely described in the stat block. Lich (Eladrin Wizard) Level 24 Elite Controller - about equivalent to a 5e lich - 5 powers, all completely contained within the statblock. Sorrowsworn Deathlord - Level 28 Lurker (leader) - 3 powers, plus an aura.
I'm pretty sure that too complex was NEVER a complaint about 4e stat blocks.
Now, to be fair, I'll freely admit that I don't mind monsters being a bit more complex than they were in 4e. That's not really a problem AFAIC. 4e monsters really were very, very simple sometimes and a bit more variation isn't necessarily a bad thing.
But, again, WHY do I need TWENTY FIVE spell slots (the A5E Mouthless One example that's being used), plus EIGHT at non spell powers, plus a reaction, plus FIVE Legendary Action options? I'm sorry, but, no, that should not be the baseline we're working from. There's a pretty broad, excluded middle here that I think that WotC is hitting pretty well with it's version of Vecna. It's significantly more complex than a 4e stat block, but, at the same time, significantly less complex than the stat block could be.
I'm seeing this as a pretty healthy compromise.