Refer to the Zealot, Stalker, Immortal example I gave above. Just because the Immortal is overpowered and the Zealot is underpowered, doesn't mean the Zealot still can't be useful for some things.
Or in the Overpowered Rock, Paper, Scissors example. Just because Rock is the best, doesn't mean nobody will ever use Paper again (in fact Paper counters Rock, so you're probably going to need it).
I don't contend that the Fighter isn't useful in some very niche situations, I just don't think that's proof that it's balanced or where it needs to be (nor that the Bomber is where it needs to be). Just my personal opinion though.
I think all 3 Triangle Ships should be equally useful for different situations, not just having Fighters as a nice alternative to some strategies where you need a cheap meat shield.