By the base numbers, it is. But when you look at what is possible for a surge to accomplish vs. a single spell of 6th level and higher, especially 8th and 9th, the surge just doesn't compete IMO. Surges might wreck a creature or two, those spells shift the dynamic of the entire encounter.
Yet the fighter gets those 4 attacks every round, the caster might get 1 or 2 of the big bangs off in an encounter.
As much as I hate the "typical adventuring day", with 6-8 encounters at 2-3 rounds each, you average about 18 rounds.
The fighter has two AS for 8 attacks in two rounds, plus 4 attacks for 16 rounds, a total of 80 attacks.
Using their highest spell slots, a wizard would use all their spell slots but 1st level and some 2nd:
9, 8, 7, 7, 6, 6, 5, 5, 5, 5, 5, (two extra from Arcane Recover), 4, 4, 4, 3, 3, 3, 2; for a total of 91 spell levels.
Improving fighters to an Extra Attack (4), would bump them up to 100 attacks in the 18 rounds, while scaling it to the minimum 6 would be an astounding 120 attacks in 18 rounds.
If you can equate how 1 attack = X number of spell levels or 1 spell level = T number of attacks, you could decide just where you want the number of attacks to sit. Personally, I think 5 would be ok, but IMO 6 would be a bit much
It's not the number of surges or number of attacks a day.
Which is a higher growth in power.
- A 1st level spell to a 9th level spell
- Attack action to Attack action Extra Attack 3 Action Surge Attack action Extra Attack 3
They didn't do so for fighter stuff. The classes features barely had any limits but no double action surge. The fighter clearly doesn't grow in combat power on the same curve that caster grow in supernaturalness.
If the fighter grew in combat power that same rate D&D spells grow, how many Attacks, Second Winds, Feats, and Action Surges would it have?