Yeah but if you could just craft pylons out of pre-hardmode materials and not need any specific requirements to make them work, that would just make teleporters obsolete. Why would anyone go through the hassle of setting up a teleporter if they can just craft themselves a pylon and throw it down anywhere. Even if you can only place one per biome, you don't really need more than one per biome. Monster farm underground, jungle arena in the jungle, ocean arena in the ocean, etc. Pylons nudge players into being involved more in the world. I'm not telling you to use teleporters. NPC happiness just allows you to get items that act as teleporters but under certain conditions earlier on, if you don't want to bother with building more towns, there is no downside in playing like you used to.Pylons make it easier to navigate around the world, not just NPCs.
Again, my idea is to use magic mirrors which are not a hardmode item. Just that you could also use a hardmode item.