At what point does defensive buff stacking just become overkill though? As I DM I already tend to have monsters switch targets when they learn that an individual is buffed in such a way that their attacks aren't as effective.
The issue wasn't only with the ability to stack buff but, apparently, because at high-level the working strategy against BBEG was to assassinate them by teleporting after a heavy buff session. Few things could withstand an alpha strike. That's why they both made magical movement more difficult (Teleportation is 7th level when it used ot be 5th) and by making buff stacking more difficult. The big reduction of spell numbers in 5e already took care of part of the problem (with many source of AC improving spells, each stacking, you could become immune to physical attacks in 3.5, while it's much more difficult in 5e). So maybe we're seeing an overkill reaction to this problem. The strange thing is, since apparently, high level play is very rare among the player base, I am not sure it's much of a problem right now... Back to the topic at hand, let's see the example above of
stacking :
Stoneskin (grants resistance to normal weapon, 4th level spell)
Mirror Image (idiosyncratic mechanism with a chance to avoid damage thrice, no Concentration BTW, 2nd)
Blur (enemies are at disadvantage to attack you, 2nd level spell)
Fire Shield (attacker hitting you get fire/cold damage, no concentration, 4th level)
Protection from Evil and Good (situational enemy are disadvantaged to attack you, 1st level)
As disadvantage don't stack making Protection useless, Fire Shield and MI stack already with either Stoneskin or Blur. So in a over the top example (requiring 2 4th level spells, 2 2nd level spell and 1 1st-level spell), the mechanical problem stems from combining Stoneskin and Blur.
(Side note: I am starting to think Mirror Image is a better spell at 2nd level to prevent damage on a caster than the 4th level stoneskin since it works on any source of damage, have at least 50% chance of avoiding damage before being spent [so it's like a 50% reduction]... unless the caster can make 3 concentration rolls in a row, I'd say MI is better, what am I missing) ?
Stacking invisibility and stoneskin isn't much better, since Inv. is arguably LESS powerful than Blur... I am surely missing example of "munchkin stacking" but I can't see them with the compliment of 5e spells.
My main problem with Concentration is with utility spells and enemy debuffs maybe, not with party buff.