Here, you conveniently ignore the HP risk exposure of AoOs and other threats that come with melee range fighting, the massive to-hit penalty fighters take in exchange for that extra damage, you take one of two martial weapons - which are both two-handed mind you - that deal 2d6 damage as the example, and assume that the surface Firebolt ignites will deal damage to only one target. You have to idealize the fighter to compete with an at-will cantrip that is neither gear nor resource dependent.
5E is a carefully balanced game. There are almost no conditions that modify to-hit, and AC spells are few and carefully tuned. Introducing stuff like this might seem fine in EA, but multiclassing will introduce an entirely new level of broken class balance that stems directly from these kinds of thoughtless modifications to the system.
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.
