Super minor quibble: the design assumption for 5E is 2-3 Rounds, but much closer to 2 Rounds than 3 on average. Which is what lead to them realizing after 5 years that there cam be a mathematical transparency of Monsters and Spell slots which undergirds the newer design choices.
Well, sure. Potato, potahto. Whether you peg things at 3 rounds or 2 rounds, probably doesn't make an enormous difference in monster design. 3 rounds vs 5+ rounds though DOES make a big difference. A 3 round monster doesn't need more than 4 things it can do. A 5 round monster needs 5 and probably a couple more because the combat will go longer than 5 rounds very often.
Thus, in 4e, you get monsters with one main action then 3-5 subsequent actions that they can choose from. Works very well for that system.
What we really don't need is a full spell list with 20+ different spells for a creature that's likely only going to last 5 rounds at the outside. Which is why they're going with this new stat block. Since it is perfectly acceptable for non-casters to completely ignore PC build rules, there's no particular reason why casters have to. Coupled with the fact that most of that stat block is just wasted space, confusing to use and often leads to DM's making mistakes during the game, there really aren't too many good reasons to keep the full spell list and lots of very good reasons to go with a truncated list.
@Voadam - that would answer your point too about looking spells up for monsters. Sure, you might have to look up Dominate Monster. Fair enough, it's something that is pretty complex to use and likely not something people have memorized. OTOH, it's not like you have six different spells that you have to look up - which is what you have in the longer spell list. Do you know how Prismatic Wall works without looking it up? Now, we've got an 18th level wizard, so, he should have a couple of 8th level spells too - let's see - Antipathy (I have zero idea how that one works without looking it up) and Control Weather.
Sure, I'm picking random examples but, I think I'm making my point. In a spell list of 20+ options, there are MANY spells the DM is going to have to look up in the middle of combat or make mistakes. Do the spells require concentration or not? What do these spells actually do? So on and so forth. And that's just spells from the SRD. Never minding if there's spells from other books as well. O.O
I'm still having trouble thinking that a longer spell list is somehow better than a truncated one. You're just not gaining anything with the longer spell list.