But it wouldn't be as dramatic as having him shot in the throat so he can't speak. And, even as a montage, it would lessen the urgency of his situation. Dr. Zaius just sits back and give Taylor weeks to learn their language before trying to having him lobotomized and gelded?
"Oh, this human is gradually learning our language. Nothing to worry about, I'm sure."
And Taylor has the leisure to take daily Ape-ish lessons from Zira, as part of his daily routine? That doesn't exactly convey a sense of danger or suspense. . ..
"It's a madhouse! With vocabulary lessons!!!"
At the end of the day, the UT is no more implausible than, say, telepathic green-blooded Vulcans, or beings who evolved a long time ago in a galaxy far, far away looking and talking like Carrie Fisher. It's just a staple of science fiction and fantasy.
If you want absolute realism, watch DEATH OF A SALESMAN.