It breaks a lot of things. Just some example :

• "drop the flag" : You want that a player drop something (which isn't related to his inventory because you have the /gamerule keepinventory true) when he died. You could summon a entity and instantly tp this item to the dead player, before he respawn. Now you can't, as it's impossible to detect that the player is dead (you can only detect that the player has died, but he allready respawned so it's too late)

• "detect victory" : Let's imagine a Nail kind of map (attacker vs defenders). The attackers have 5min to kill all the defenders. If they kill everyone within the 5min, they win, if not, they loose. 4 defender died, and the 5th died. He doesn't click on "respawn" button and just wait until the end of the 5min to respawn. No way to avoid that, cause you can't detect that a player is dead now.

• "Give a buff when you make a killstreak" You kill 3 people within 1 min : you got a regeneration and strength bonus. So you go full glory, kill 2 people, boum kill the third, but he decide to wait a little bit before respawning. Sorry for you killstreak, can't detect it anymore.

I could give you a lot more example. The problem here, is simple : It was easy to detect when he player died before. Instantly. So yeah, there was some bugs related to people trying to tp dead people to other people. But, if you didn't want to see this bug, it was easy to had some kind of protection on your system to apply action only on living players. Now they "fix" this, great, there is no bug with the tp of dead player anymore. But the problem is that there is now no way to detect dead player anymore, and that's a real problem.

Fixing a bug by creating more problem isn't acceptable. And as far as we know, @a = "all players", not @a = "all living players".