So why are those three elements the cornerstone for Destruction magic? Why haven't there been any spells for Light (which the Ayleids thought were a pristine stage of fire), liquid Water, Earth, or Wind elements within the School of Destruction all these years? I know there has been daedra and thu'um associated with some of those elements, so they clearly exist.

And why did they ditch the acid/poison element from the spell selection? I guess the Mages Guild were really falling apart at the end of the third era.

Dragonborn DLC added few destruction pells not associated with Fire/Shock/Ice. But thae answer for your question is: Game Mechanics =/= Lore.

There are wind and poison spells in Dragonborn.

Light, water and earth manipulation falls more under Alteration, IMO.

There are Restoration spells in Dawnguard that do Light Damage, which only affects undead.