Hussar
Legend
Why? And I don't mean this snarkily. I mean this as a real question. What function would having 40+ known spells listed in his stat block?He should have all the spells and spell slots of an 20+ level arch mage.
But there are many many DM's who DO have the experience and organizational skills to run complex spell casters, especially ones that were historically spell casters first and foremost. These changes badly hurt their options, and adds a ton of DM prep time as DM's now have to add them back in (or simply ignore them and stick with the real Mord's...yeah, I have a a copy of that and it will never go away).
And, we're right back to the appeals to the unknown mass of DM's who apparently can do this. Never minding that there is a preponderance of evidence that shows this not to be true - live plays and actual recordings of play demonstrating that even experienced (and some VERY experienced) DM's can't run complex spell casters without making mistakes. Never minding that people have been complaining for YEARS that high level play is too difficult. Never minding that 5e has already rejected the idea of forcing monsters to follow PC design rules. Apparently, there's this silent mass of DM's out there who can perfectly run these creatures without error.

