If we assume that the homegrown troops had not opted to resist Alexander, then I believe there would have been a series of battles in rapid succession. Alexander, having learned from his experience against Porus would have become better and better against the Indian armies.
But eventually he would realize that India was not really worth it. Unpleasant and far between riches (oh yes there were riches). He would sign agreements similar to protectorates and then head off to fight the Chinese instead.