I love being able to cast two spells per a turn, so I definitely wouldn’t want that cut! It makes combat much more fun for me to be able to use my spells!
Casting two (or more) full action spells in a single turn is on a whole different power level than casting one action plus one bonus action spell of any level in the same turn, which is itself on a whole different level than casting an action and a bonus action spell but one of them is restricted to a cantrip (this last one is 5e RAW**).
If Potions of Speed didn't let you cast a full-action spell with your extra action and Quicken didn't allow you to cast two normally-full-action spells in a turn, then the massive exploits mentioned above would be fixed AND you, Icelyn, would still be able cast an action and a bonus action spell every turn. Is that not enough? At some point, the game has to put in restrictions on what you can do in a turn; otherwise it loses all challenge and essentially becomes an interactive movie. (The dedicated "Story" mode difficulty should be present if you want this experience.)
**The one valid way to cast multiple spells in a turn is to take 2 levels of Fighter (or just be a pure Eldritch Knight) for Action Surge. RAW, this additional action allows you to cast another full-level spell with it, but at the cost of being 2 levels behind in spellcasting progression compared to full casters. An appropriate tradeoff.
