Yes the number of different battlefields are finite. According to Lar the amount is huge though. There is a difference between revisting and replaying. Replaying means you start from scratch and thus you will in all likelyhood end up with differently generated battlefields. When revisiting there is a limit to how far you can level up. Although we do not know how big one of those areas is and how many gaming hours it takes to clear out the area. Neither do we know how many rifts there are to access and if we learn how to create our own rifts, if we can continue doing that after the game has finished. If all of this can be answered positively then you can just keep on creating new rifts and thuse go on until you have exhausted the finite amount.
Lar did mention that monsters do not respawn at the time we asked him. If this remains is unsure of course. At the time we asked him the quest generator was just abandoned, so that migh be a reason to reintroduce respawning of monsters, although I don't like respawning personally, just like I don't like games that never end.
