I think 1 million is very reasonable and there is a great possibility of more. I played vanilla starting in 2004 and the truth is I have no real interest in playing classic. Yet, since it is free w/WoW sub and because of the nostalgia of it I will certainly be there at launch and play for a while.
However the test will be over the coming months. The original WoW was amazing as subscribers quickly grew from 1 million to 2 million and then to 4 million. However, I suspect that whatever the launch numbers are the numbers will fall this time as the nostalgia wears off.
There were a lot of player demands for the changes that came in later versions of WoW. I remember how much we all loved what was a unique experience back then but I also remember how totally frustrating much of WoW could be. There were far more changes I really liked post-vanilla then things I missed.
So players who really like the new WoW Classic would do well to make the game welcoming and enjoyable for players not so enthralled with vanilla.
But it will be fun to see an undivided Barrens and an unflooded Thousand Needles again.