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.
Yes, jumps and disengagement being conflated form a very silly mechanic. If used properly by the NPCs, it would completely distort the ruleset, as 5E uses chokepoints and AoOs in lieu of more modern solutions like taunting or hard CC as a primary mode of threat mitigation. Even as is, the conflation marginalizes Rogues, who should uniquely receive Disengage as a bonus action.
You hardly have to idealize Firebolt, it doesn't consume resources, can be used at the same range as most other spells in the game, there's a flammable surface in almost every fight, and even when there's not, castings, scrolls and bottles of Grease become both an AoE damage spell and CC, which is a pretty significant buff. It's pointless to consider these things in a vacuum with regard to breakpoints and outperformances, and far more useful to compare them to their tabletop counterparts, because cantrips and attack actions are embedded in a more complex system that is balanced around them. If you move one of those pieces, such as buffing Firebolt, all the other pieces are drawn along with it. As an example, I don't see Wizards losing a spell slot later on to service this early-game buff.
How about a single level dip in Fighter producing the Firebolt damage output we're talking about here at Level 2 while still being able to wield a shield for +2 AC, while the melee fighter being touted is swinging a 2-hander. A +2 AC advantage in the number-crunched 5th edition is a pretty sizeable buff.
