Will NPCs be throwing out 40D6 meteor swarms (140 points of damage) at level 17 when the PCs have 100 to 250 hit points? How about if it is a Sorcerer with metamagic like Heightened or Quickened and they throw out another high level damaging spell simultaneously? If so, the game could end real quick.
Will most every player be taking the Tough feat, just because they lose one third to two thirds of their hit points in the first round of combat several times a day when facing enemy casters?
I definitely get what you are saying. But 5E monsters appear to be able to cast the same spells that PCs can (based on the stat blocks we have seen so far). Whenever that happens and a PC spell is a bit too good, it's a bit too good for the monsters as well.
What happens when the high level monsters start casting Mass Heal?
If you think this is going to be an issue in your games, then why are you giving your NPCs Meteor Swarm and Mass Heal in the first place?
You're speaking as though you have no control over what your NPCs will have available. You're the DM. You control what spells they have (if any) and you control how they are used. This is part of the reason why monsters are no longer built like PCs but are given their own unique abilities-- so that those abilities are designed and balanced against what the PCs will have. And I would be very surprised if any monster in the upcoming Monster Manual will have 9th level spells available in their statblock just because of the potential unbalancing unless the rest of the monster's block has been designed to compensate for them.
We're not going to see any monster or NPC in the MM that looks exactly like a 20th level Wizard (with all 22 Wizard slots available to it). And that was done purposefully as part of their design paradigm. So for DMs to deliberately go against that paradigm and build their NPCs as PCs in the expectation they will fight the PCs means they're going to have adjust their combats accordingly. Does that mean needing to have the 20th level Wizard NPC "fight dumb" to avoid killing the PCs? Perhaps. Or do the PCs at the table have to accept the DMs credo that whatever's good for the goose is good for the gander? Perhaps. But in any event, it's the DMs choice and decision to play the game in that fashion, and accept the consequences of that decision accordingly. But you can't then go blaming WotC for "allowing" it to happen.
It's not their job to save a DM from his or herself.