I’m sure some people are positive which realm they’d want to play on.
I am also sure some would like some time familiarizing themselves with the game before making a decision which realm to play on.
I think they’re very unlikely to have 2 realms upon release. 1 full ladder or 2 I think is more than reasonable to add personal loot, charm inventory, drop rate changes, runeword changes, unique/sets, etc.