What makes you do this if you don't want to?
Nothing stops me from throwing together my own monster stats without going through the whole process. I do it all the time.
For example, I'm going to be running encounters next week in Pathfinder that involves three monsters from the Bestiary and one monster with class levels added. I already have all of their stats, literally sitting in front of me, but I'll be restatting all of them to be easier to run and more distinctive.
Now, I don't doubt that Paizo can produce cool, interesting monsters. But, because of the way 3.5 monsters work, you end up with this huge mess of stats justifying the stats that you're actually using and then a bunch of other stats based on those justifications so you get... garbage, really, or at least more noise than signal.
That's what I don't want to see in 5e. I don't need them to justify the monster's stats to me. I just need the monster's stats.
The real problem I have with your statement is the use of the word "shouldn't". That implies that there is a way these things ought to be done. I don't think that this is an issue with an "ought". You do what you need to do to achieve the result you want. If you need to make up something on the fly, you do something simple and quick that doesn't create a lot of overhead. If you are planning out a boss fight that is supposed to be the climax of a major story arc, then you invest as much detail into that encounter as you can manage.
Well, I'm not claiming any moral authority there. I don't mean "shouldn't" as in "you're a bad person if you do this," just "shouldn't" as in "I think that's a bad idea."
But I do believe that building enemies using the PC rules reaches the level of being a bad idea above and beyond personal preference. Using other techniques, you can build a more interesting enemy faster that will run more easily in play.
You may disagree on whether that's the case. But hopefully we can agree that people "shouldn't" do more work to get worse results.
But in terms of allowing you to wing the fights that matter, I don't feel at all that it was easier to do (good) 4e design than 3e design as something like my stat block for an epic boss fight with Tharizdun shows.
I can't speak specifically towards your Tharizdun fight, but I'm familiar with your slaad lords (which are awesome, by the way). At least with those, I agree going 4e doesn't really change anything, but you're also not building them using classes that I remember.
Hand-crafting epic boss fights is the way to go. I won't disagree with that at all. Far from it.
But I don't think building the boss with PC classes will make the fight better (even if the boss is nominally a wizard or whatever). In my experience, you get a ton of abilities you don't need and, after all that work, still have to add the features that make the fight epic.
Cheers!
Kinak