I'm genuinely curious what you (or anyone else) would actually propose WotC do to resolve this.
I'm anyone else, not Dave, and my suggestion would be an "Advanced Monster Manual" where the design assumes
1) the feats in the Player's Handbook
2) the multiclassing options in the Player's Handbook
3) the level of magic item the actual treasure guidelines of the DMG result in
In other words,
nothing out of the ordinary. Just using the product features you have paid good money for.
By the way:
a) I think the proportion of gamers that run "option on" games is way higher than 10%. In fact, I think that figure is just plucked from the air to help apologize for WotC's stance.
b) This manual would obviously be of use for all players, not just options-on groups. If you so readily accept "you can always make the monster harder yourself" and "simply use this with groups of a lower Average Party Level" then you should have no trouble accepting a short blurb in its preface saying "this is for advanced players that have mastered the game's options. You can always consider it's challenge ratings to be 25% or even 50% higher if you have a weak, uncoordinated or newbie group". And of course, it should not just merely duplicate existing monsters - to be of real value, I propose that at least two-thirds of it offer monsters not previously seen with 5E stats. (As for new versions of existing monsters, demons are particularly egregious offenders.)
Thank you for asking.
Edit: what are the basic changes needed, then?
* a fixed action economy. Only those monsters that would otherwise be unbeatable should retain a Teleport action as your standard action, for instance. Every other monster should be able to pull off its main attack sequence AND be able to do other interesting stuff in the same turn.
* a fixed skill section. Around 10th level player characters become unbeatable as regards skills. If you focus on a particular skill you realize no monster can match you, let alone surpass you. This feels exceedingly wonky. Of course a monster described as a "stealth master" and an "expert ambusher" needs to out-stealth a level-appropriate party. This means a CR 5 monster should probably have a +15 bonus at the very minimum, since all min-maxed parties will contain at least one character with passive perception of 17 or higher.
* roughly twice the hp
* robust support for Solo creatures. This includes a second doubling of hit points, and at least twice the actions (of all kinds) given by the Basic Monster Manual.
* at least some effort at Epic rules (I guess at CR 21 and above). A real Archmage NPC needs to be able to break the rules on concentration, attunement and everything else. As a good example of how to fix the action economy, introduce an Epic Time Stop spell which isn't restricted on what actions you may take.
Just scratching the surface here, so you can get an inkling of how large this AMM task would be and how valuable it would be to be able to purchase a finished product instead of having to do it all by yourself
Edit 2: if the mods feel it's warranted to break off this sub-discussion into a thread of its own to keep the focus here squarely on the monster, no complaints from me.