Consider a single TIE Advanced vs two TIE Fighters.
Both options are going to cost about 24 points.
A TIE Advanced only has half the firepower of two TIEs. True, it can take missiles, but that's a one-shot deal.
Two TIE Fighters, on the whole, are more survivable with 6 HP total compared to 5 (with shields).
The two TIEs have two actions between them whereas the Advanced has only one.
The TIE Advanced does generally have better initiative.
Personally, I agree with the Empire's in-universe decision on this and they went with the cheaper, more mass-produceable TIE Fighter. "Quantity has a quality of its own." In WWII, Germany's Tiger and Panther tanks easily outclassed the Allies Shermans and T-34s, but simply couldn't help being overwhelmed by the sheer numbers of the enemy.
However, it can be reasonable to argue the other side.
You forgot to add target lock into that equation as well as the facts that 2 of those five the tie is sporting can eat crits and that after three hits the dual ties lose their advantage. Not saying your points aren't perfectly valid, because they are, but you left out a few pros to the advance tie.
Really it boils down to personal playstyle in the end, as well as how the rest of your squad is composed. Runing the buffer hero ties obviously favours numbers where as the self boosting ties don't.
To use a rebel example, Dutch really shines when Wedge or Hortan are around and packing protons, Garven works well with Biggs and so on. Squad leader vader can make good use of a fellow missile carrying advance as well since he has two actions to hand out.
All a matter of taste and who you're fighting really.