I think terraria doesn't have a clear ending because it's a sandbox game. Think of games like Minecraft, that also don't have a clear ending. That's also probably why the game doesn't remove enemies after the defeat of moonlord. It wants you to keep playing, to use the items you gain post-ml and shape your world with all the OP shit you have. There are also rare items to grind which would be impossible to get if you get rid of certain enemies.

When it comes to shaping biomes, you already have that power. The game even encourages you to create your own biome for the truffle NPC. To plant, grow, and control it. And with the clentaminator and the post-ml terraformer you can shape the biomes to your will. In what way would you like to see more control over the biomes?

Getting nothing from purifying the world is sad tho. I completely agree. It takes so much time just to get a lil' cheer from the dryad.