Before the airtoss patch you could watch almost any professional match and protoss players would regularly lose control of parts of their army. I can’t tell you the amount of times I cringed in despair as the best protoss players in the world lost random disruptors / high templars because they went a different direction than the rest of the army. But why is that, because the optimal protoss army required as many units casting spells as possible.
You had:
Oracles for stasis traps/revelation
High Templars for storm
High Templars + Warp Prism to protect HTs from long range units that nullified them (Ghosts, Infestors).
Sentries for Forcefields / guardian shield.
Disruptors for AoE.
Warp Prism for reinforcements.
Stalkers for large aggressive blink forwards/backwards but also the optimal 1-3 blink backs to prolong their life in battle.
Phoenix Gravitron for tanks/cyclones/immortals (not used often but has been used to great effect).
To wield the best army you had to make use of all/most of these units.
In comparison zergs have:
Lurkers burrow a single use ability before or at the start of a fight.
Infestors.
Vipers.
Right clicking banelings past marauders/tanks/thors/archons/immortals.
Ravager Biles.
Queen Healing.
But none of these units were ever all required to be an effective zerg army, I think at best you need half?
Now compare that to bio for terran:
Marines/Marauders need to press stim, all together which is the vast bulk of the terran army.
Tanks to siege, a single use ability before or at the start of the fight.
Liberates to siege, a single use ability before or at the start of the fight.
Widow Mines to burrow, a single use ability before or at the start of the fight.
Ravens for armor reduction.
Ghosts for EMP/Snipe.
Simply put the protoss army required more individual moving parts than any other which regularly meant the protoss armies would just fall apart after a split second of diverted attention and once they lost the key component of their army the rest of the army would fall behind.
This also fails to take into account that stalkers are one of the most apm intensive units in the game as to “optimally” play them you need to be blinking them back individually something that only Alphastar was able to do.
What all of this means is that Protoss were left with cheesey all ins the majority of the time, the APM demand of a late game protoss army vs zerg and terran was ridiculously disgusting.
Protoss sucked before because their armies required higher degrees of micro than any other army. Which is Blizzards fault, Blizzard gave Protoss Colossus but that just resulted in massive A move armies so they nerfed the hell out of them and gave Protoss Disruptors instead which are literal clunky AoE deathballs that require babysitting to the nth degree.
So if you want to fix protoss give them an AoE that isn’t countered by a single unit but also doesn’t require tons of micro either. A mix between Collosus and Disruptors.
Edit: I want to add that when referring to APM/Micro demands for PvZ or PvT I’m purely talking about the spell casting portion and not the splitting of marines/lings/etc.