I think that would make sense if the goal was to make a language optimized for understanding by hearing people or late-deafened people, or anyone else already using English. But I think the goal was to provide for deaf people in a way that was optimal for them, that was best suited to the way they would naturally tend to communicate.
I understand what you're saying: why bring/develop a new language for a community that lives within an English speaking community, so that the larger community and the smaller one can't communicate easily, right?
And I think it's a valid concern, but much more important is that deaf people have a langauge that is naturally suited to communicating manually.
Especially since so many ASL users are very good at using/understanding PSE, so if all you learn is how to sign in English, you can still communicate with them. Nothing really lost, and the Deaf community has a strong language, not something patched together awkwardly from another medium.
