I personally know 3 people who converted to Catholicism after WYD in Sydney. My brother in-law met his American wife at the previous WYD.
There were more from around Australia, New Zealand and the Pacific region who came to see what all the fuss was about and were converted.
My friends had various reasons but all agreed that all of their prejudices, misconceptions and doubts were demolished during the Q&A sessions with various Bishops. I don’t know why because in some cases the Bishop gave the same answer I did.

I suppose being a Bishop carries more weight.

On other questions they said it better, simpler or their delivery was better heard.
It brings the Church from Rome to the people, we got to see exhibits normally only seen at the Vatican and learned so much in those two weeks.
Imagine meeting Catholics from Iraq, Pakistan, India, Palestine, Israel, Lebanon, Egypt, South Africa, Ghana, Zimbabwe, Fiji, Singapore, China, Russia, Mongolia, Peru, Cameroon…and so on. It was such a great learning experience for young Catholics to learn about their cultures and their peculiar flavour of Catholicism.
I’m sure some Traditionalists look down their nose at us who participate in such events but I’ve only heard of and experienced positive outcomes. Whenever a Mass is celebrated that is something special.
If you were a young Catholic who was only nominally Catholic, you will come away from this event changed and reinvigorated about the faith.