If he did that, the main thing that would change is the conversation from "Is God real" to "Should we follow him?" Granted, that's a pretty large question but God would still have his followers and non-followers. He would get a significant boost in people that believe that he exists, to say the least, but not necessarily the same boost in people that decide to follow Christianity.
There is an unconditional love that comes with being a parent/child. Children are allowed to make their own choices but at the end of the day, as a parent, you're responsible for your children and their well-being. Part of being a good parent is showing up when your kids are in need. If I was never around to take care of my child and said, "I'm with you in spirit even though you'll never be able to physically see me," I'm sure they would grow up to resent me at least a bit.