Well part of the problem is that they got rid of SR:Yes/SR:No spells that allowed a boss with SR to make save or lose & the most eyewatering damage spells very hard to land. Now instead of casters switching to more cooperative & reciprocity leaning SR:No spells 5e just has the obnoxious diceroll the save->fail->"nahhh legendary resist" that ultimately feels bad for everyone unless it's left still bad & the gm leaves them unused.We shouldn't be blaming Counterspell for the fact that WotC can't Boss Fight their way out of a paper bag.
If your BBEG fails because they failed to get one single spell off, then they were either going to fail naturally or TPK with their big unfairness spell.