Cap, it feels like you're unnecessarily restricting yourself here... I appreciate your perspective that it would be helpful for WotC to put more stat-blocks of fully-developed NPCs with class levels into their published adventures, but simply because they haven't done so to your satisfaction doesn't mean that they don't exist, or that you shouldn't feel entitled to create them...
Specifically, in the DMG on page 92, it encourages DMs to use monster stat blocks for NPCs and/or give NPCs a class and levels...
I have not played in or run a D&D game in the past 20 years, where NPCs didn't have classes and levels.
Additionally, I have never perceived published adventures as anything other than a basic template or blueprint laid out for the DM, upon which the DM can add or refine elements and tailor it to challenge a particular group of PCs... If 5e was the only edition of D&D that ever existed, I might be more inclined toward your approach. But, when taken within the context of all editions of D&D... Whether it be TSR, WotC, Hasbro, etc... They provide you with the blueprints, materials, tools, etc... But it's up to you to build the house, tailored the way you like it, after all, your group is the one living in it...