Those "barriers" were important balancing elements that got discarded on the altar of simplicity in service of more simplicity for players only.It's two fold.
The spells were simplified and thus removed the barriers and limitations D&D's powerful magic could run into.
Again they served as both a defense for monsters in addition to a positive incentive for players to choose different more cooperative spells over encounter ending ones that leave others feeling useless if the spell lands.The monsters were simplified so they lacked the raw numbers nor additional defences they needed to resist D&D magic outside of case by case basis.
This isn't describing anything that should be be considered an improvement. If simplicity is an improvement, then the benefits of that simplicity should be self evident enough to stand on their own, the 5e legendary resistance with six trivially targeted saves and widely applied god wizard type debuff/control save or suck/lose/die spells falls very far from being able to stand as an improvement once there is more than one player who's experience matters.So monsters had to be clutched on the back end by giving them legendary resistance spell resistance and various saving through a proficiencies just to be valid boss monsters.
You couldn't simply grab a monster and use it as a boss they had to be designed as a boss in order to have the wrong numbers needed to be boss worthy. There was no real way to calculate how strong giving a monster legendary resistance and spell resistance on the back end from a blank state.
You couldn't simply give your fire giant legendary resistance and spell resistance and have a good handle on how strong that monster would be and would be mostly winging it.