Ok. I like Strongholds and Followers but to say it’s much better than anything WotC releases is surprising. It’s different, but a lot of people would consider it overpowered and pretty niche. I’d certainly have to think carefully before including the options, the one time it fitted and I did, it worked.
I don’t see how you can compare a supplement with a few subsystems though to something with the complexity of a campaign book. You literally just said you don’t want drop in stuff that’s too short but somehow where evil lurks (which is full of such things) is spot on? The difference in page count is like two or three pages per adventure.
I don't recall saying everything MCDM produces is better than everything WotC produces, so I'm sorry if I gave that impression. I did say Matt Colville worked on Strongholds and Followers, and I think it was a pretty high quality offering generally speaking.
I would say that his Flee Mortals is a higher quality than the Monster Manual (for my tastes). The organization is better and more useful. It has subsystems (companions, minions). It has what I think is a workable challenge system, and the art/paper quality is better too (IMO).
So let me compare Where Evil Lurks to a "lair" book (like Candlekeep) - and this is just my preference. First, Where Evil Lurks has a small dungeon of 5+ rooms, chock full of encounters and treasure rewards. Each will easily last an evening of play. This is also because the fights are more involved and dynamic than typical 5e fights ... whereas Candlekeep might be "go here and there's a Knight - use the stats from the Monster Manual." Candlekeep uses its page count for fluff and non-gameable content, where as Where Evil Lurks is packed with adventure, monsters, traps, treasure, maps. Where Evil Lurks is 4-5 hours of play. Candlekeep is "maybe" 1-2 hours?
Where Evil Lurks also spans Levels 2-20, with something for every level (some levels have multiple adventures too.) Candlekeep has levels 1-16 (because WotC rarely writes above the teens?)
So, yes, Where Evil Lurks presents drop-in "monster of the week" lairs, they are more fleshed out than what we find in a product like Candlekeep Mysteries. (Additionally, it provides new monsters from Flee Mortals and the relevant rules and subsystems.) I think it's a much better value and more inspired design, at least for my tastes.