Gangplank, Tristana etc. Balance of AP abilitys

Hi, why do champions with really high physical damage potential have such high base values on their AP scaling abilitys?

Gangplank for example is really oppressive in the late game. He does insane damage with his AAs, Q and E, but also has a high base values on his AP scaling utility spells. A way to make him more healthy would be to decrease the base value of his cleanse/heal and his global ranged slow, while increasing the AP scalings. This would not only balance AD Gankplank but could open up AP builds paths too. Riot nerfed the base damage of Miss Fortunes E too, so why can Gankplank have such a high base damage on his ultimate, without having to give up some AD?

Another case is Tristana. Why does a hypercarry like Tristana with insane Autoattack damage has high base damages on her AP scaling utility spells? AD Tristana uses her jump and knockback for damage like an assassin. The base damages are so high and the AP scalings are so low, that it is not worth to build AP. If the base damages would be lower and the AP scalings higher, AD Tristana would be more balanced and AP Tristana would be worth to play.

What I want are different build path options with different strengths instead of one overpowered build path without weakness.