If what we are discussing is correct and the studio can't afford having Yamato's actress as a regular, then that probably means Oda never, ever intended Yamato to join the crew so he "allowed" Toei to choose a superstar. Hiring a new actress would be the worst thing to do right now because Yamato already has a established voice and such changes only happen for major reasons (great scandals like what happened to Ivankov's first voice, sickness or death).
Anime is commonly treated as a secondary product; One Piece isn't the only series which received a "second class" dubbing. Dragon Ball too may be the best example of it, with a cast most of which has either done little dubbing or were more popular as screen actors.