If we get modding support on the scale of Minecraft Forge (rather than tAPI, which sadly is not as effective), they could actually be handled using separate mods, for those who don't really want one or the other.
As an additional note, there was mention earlier of sandbox games requiring a constant stream of new content to survive- and while this IS true, it doesn't have to be official content. I'm going to be that one
and compare Terraria to Minecraft (solely on the basis of they're both sandbox games, and NOTHING else)- if not for the fact that mods exist, I would have lost interest in Minecraft a LONG time ago, and I know I'm not the only one. For a lot of people, vanilla Minecraft's pretty boring (it's really shallow tbh), but with mods it gets REALLY interesting. While that entire thing is not necessarily true for Terraria, the point of mods maintaining interest in the game still does.
In fact, Steam Workshop support would probably dramatically increase the amount of interest in the game, not to mention make it easier for modders to be, well, known. (Mods aren't as well-known in the Terraria community as they are in the Minecraft community.)
Oh yeah, and before I forget.
The concept of a fourth set of biomes terrifies me. I'm afraid of whatever Tobbvald has planned.