How can Hinduism and Buddhism remain the predominant religions in Indonesia in otl?
Emm... they do ?
When the Portuguese arrived Islam wasn't surely rooted in Indonesian culture, however, Muslim states dates back to, at least, the 1300's IIRC. In my opinion, if the Portuguese occupy all the centers of Islamic expansion in the Archipelago Hinduism Buddhism and Native religions will probably be relevant ouside the urban areas, where of course Catholicism will be prodominant.