Increasing your damage by 50% is useless? When most campaigns don't get above 10th level, I think people underestimate the off-hand attack for fighters.
Ok, let's ignore everything that happens above 10th level, for the reason you cited.
A 5th TWF level fighter (standard array) will have 16-17 STR or DEX, TWF fighting style and the dual wield feat. This fighter, using its attack and bonus actions, will make 3 attacks, with +6 to hit, each of them doing 1d8+3 damage. Against an average of AC 13, that's a DPR of 16,875.
Let's make a 5th polearm master level fighter (standard array). It will have 16-17 STR, defense fighting style (mostly chosen for ease of comparison) and the polearm master feat. This fighter, using its attack and bonus actions, will make 3 attacks, with +6 to hit, two of them doing 1d10+3 damage, one doing 1d4+3. Against an average of AC 13, that's a DPR of 16,875, the exact same of the TWF. They also have the same AC (dual wield vs defense style). The Polearm Master, however:
- has reach
- can make opportunity attacks when enemies enter its reach
- has a slightly more damaging opportunity attack, in general
- its action surge attack action does more damage
- if both characters find a single powerful magic weapon, the polearm master will benefit more from it
I'd argue the polearm master is ahead of the TW fighter, and would get more ahead if higher levels see play.
Let's try with an hypothetic greatsword user. It has have 18-19 STR (it picked ability score increase at 4th) and defense fighting style (again for ease of comparison). This fighter, using its attack action, will make 2 attacks, with +7 to hit, each doing 2d6+4 damage. Against an average of AC 13, that's a DPR of 17,6 slightly more than the TW fighter. They also have the same AC.
This Greatsword fighter, however:
- doesn't have to waste its bonus action every round. It can freely second wind without losing damage, for example
- has a substantially more damaging opportunity attack
- its action surge attack action does considerably more damage
- if both characters find a single powerful magic weapon, the greatsword user will benefit more from it
Only thing the TW fighter has over this greatsword fighter is, I believe, less wasted damage on overkills. I'd still pick this over the TW fighter everyday tbh (and so have all my players for almost 10 years of campaigns - rogue excluded ofc).
If I'm building the TW fighter wrong, please tell me, but what I'm seeing is it falling off as early as 5th level.