Not that I engage in debates with organizations via e-mail, I thought the same as your earlier point about the first members were converts as well but I didn’t really want that to be my first step. It’s sort of like suing a fraternity for membership.
I used to debate them about the conversion issue; though I felt I could do that since my mother was born a Zoroastrian. After many discussions, finally came to the conclusion that they often confuse their ethnic identity (being Parsis in India) with being a Zoroastrian. The two are not the same. Whilst no one can become a Parsi, the Parsis do not hold a monopoly over the Zoroastrian faith or religion. They are just one group of Zoroastrians.
That said, I can also perfectly understand some of their sentiments. For example, there are those, past and present, who deliberately wish to bring harm to the community and their faith, by different means - that’s still very real, and one reason one can’t convert unless completes a number of steps to make a true commitment; it isn’t the latest fad or fashionable thing to do.
To be fair not all Parsis are against conversion. In fact there are divisions among them too. Younger generation is much more receptive.