I'm not assuming anything. P.92 of the DMG (2014) says you can use a monster stat block, or you can create one like a PC using class and levels. Off-hand, I'm not sure there are any examples of the latter in print. The only real evidence this was ever seriously considered is the existence of the Oathbreaker Paladin.
Meanwhile, in the revised DMG, page 84, it says to use a stat block from the Monster Manual. Period. What you are describing does not exist in the rules-set under discussion in this thread. Even if you created such a thing, how would you know what CR to give it? The handy-dandy chart from the previous version is gone. Rules for creating new monster and NPC stat blocks are gone. Even if you did create such a thing, how would you reconcile the difference in Proficiency Bonus between a 12th-level Cleric, and a monster of equivalent CR?
So I'm not assuming anything. I am making the factual observation that his rules-set does not support classed NPCs. The changes from the previous version make it clear they don't want you to even try. If you have some actual advice on how to incorporate PC-style NPCs in the 5e Revised rules, and some reasons why you would do that, I'm all ears.
It is simply true that in 1e, 2e, 3e, you could create an NPC who was a "6th-level wizard" and that would be intelligible in game terms. In 4e, you could design an NPC of any given CR you want, and assign them some PC abilities to approximate a particular class or archetype. In 5e, the DMG at least gave you some napkin-math guidelines on how you might do such a thing. In the new rules, it isn't a thing. I can't even do what you're suggesting without cracking open my 2014 DMG or wildly making up numbers.
So this is completely unlike 1e and 2e in that regard. In AD&D, a bandit is a sub-1 HD monster with 1-6 hit points, who is always led by an 8th, 9th, or 10th level fighter, with six guards of 2nd-level fighting ability and a 7th level lieutenant. There is nothing even vaguely equivalent to this in the 5e revised rules. You can't even tell me what the XP bounty on a 10th level fighter is.