I actually stopped playing the game for years because of this, twice. The idea that your sandbox world can permanently (unless you want to singlehandedly rebuild an entire biome) lose variety is so incredibly anti-fun. I wouldn't care if it converted in a reversible way, like turning mud into corrupt/crimson/hallow mud, and jungle grass into jungle-corrupted/crimson/hallowed grass, etc. If it was like that then the player could always feasibly work on restoring it with the clentaminator (which is already a lot of work, and good luck if you miss any single blocks). As it stands, you essentially have to babysit the jungle for your whole game, or it'll just get progressively destroyed permanently. I like being able to build different themed structures in all the different biomes, and I really love the aesthetics of the jungle, especially underground, but knowing that it'll most likely get totally messed up despite my best attempts really hampers my desire to engage with the building aspects of the game from the get-go.