This is one of those ideas that sounds good in theory and completely collapses in execution. You're not actually "solving" the problem of CH chains and melee blobs; you're just dragging the fight out longer while keeping the same underlying design issues intact.
If anything, it makes the encounter less interesting because now you've doubled the downtime without adding any meaningful player agency. Making mobs tankier is just the illusion of challenge through attrition - there's no depth there. Players don't suddenly start coordinating better because a boss has ten million HP; they just stare at cooldowns for longer and pray the clerics don't fall asleep.
Velious worked then because it was the first time people were seeing that scale of encounter. It had spectacle, novelty, and technical limits that forced strategy. That's not the same as good mechanical design, it's just what we tolerated before we knew better.
If you want to fix "blob rush" gameplay, you have to change incentives, not stats. Add dynamic mechanics that punish zerging, create meaningful positioning, add debuff interactions that reward small coordinated groups over raw numbers. Otherwise, you're not evolving the genre, you're just polishing the same 25-year-old rock and calling it granite.
