The Fighter is more than 3 times as effective as the Wizard when it comes to fighting with a weapon he's Legendary in. Does that feel appropriate to you yet?
Firstly, that only works in your very specific circumstance. If the target's AC is lower, the fighter has a smaller advantage over the wizard.
Secondly, when I think of "Legendary" vs "Untrained" I think of a curb stomp in favor if the Legendary guy. That doesn't happen in PF2.
For example: A wizard with a DEX of 18 and a Fighter with a DEX of 18 with Legendary training in bows decide to have an archery contest. They use the same bow and the higher roll indicates a closer shot to the bullseye. 27.5% of the time, the wizard shoots closer to the bullseye than the fighter on a given shot.
Yes, certain Skill uses are only available to the Legendary guy, but in a basic test of skill, he loses more than 1/4th of the time.