AoO risk is mitigated by jumps which disengage you entirely which is a new invention as well and other risks can be mitigated in the usual way be it class abilities or consumables or positioning etc.
And while, yes, you have to idealize the fighter you will need to do the same for the caster in order to see the context in which an at-will cantrip is outperforming the fighter and see the break points involved. Then you can make adjustments be it discarding systems altogether or changing the numbers.
5E is balanced and biased towards certain things, it is not simply balanced, and it's always up to the DM to make adjustments to tilt the balance in whatever bias they are aiming for.
And please, by all means, post those broken multi classes you can find with just the PHB that's going to be on a WHOLE NEW LEVEL.
Disengage is not a new invention, combining it with jump is the only new thing there (which looks pretty stupid, but I digress). Also, at the current state of EA you don't really need an "idealized" caster to take down an idealized fighter, you could literally pull it off with three cantrips: Acid Splash to reduce their AC, Firebolt to set them on fire constantly, and Ray of Frost to knock them prone over and over. Is this an idealized caster? Of course not, it's just the simple and glaring issue that Larian made cantrips so OP that a caster can easily take down a fighter with the way they changed rules around. A cantrip that sets the enemy on fire even if it misses? That's free damage, no rolls even required. One that reduces AC just be making a giant pool of acid in an area? A free way to reduce AC, which is a HUGE deal. And finally one that knocks enemies prone more often than not, slows their movement range by 10 feet, and creates a rough terrain surface that can also knock them prone and reduces their speed yet again? Good grief, casters don't even need to use spell slots with these cantrips available. That's not even an exaggeration, I have gone through several combat encounters without even realizing that Gale (the companion wizard) had no spell slots left because his cantrips are so OP I almost never use regular spells.
