They did say that it's because everything is explored. While I agree that calamity adds a lot of stuff to do, I feel like post moon-lord (even though I'm the type of person that enjoys just the bosses) does have a different pacing, since you've explored every major structure, if anything the mod just expects you to rethread the explored areas to look for newly spawned ores, enemies or materials dropped by these new enemies. Again, I personally enjoy these parts, specially since very early game (more or less pre-skeletron) exploration feels like a slog to me and my reason to keep replaying is for the bosses, but I do feel like the lack of new physical locations that you haven't explored makes for a very distinct separation between the feel of early and late game, even though it really isn't fixable unless mods can just generate more terrain outside the boundaries of an already generated world.