It actually does slow after Plantera, significantly.I think the jungle should not be immune to crimson/corruption/hallowed, but the spread should be slowed when a mechanical boss or Plantera is defeated. If a dirt block is surrounded by 2 or more jungle grass, I think it should convert the dirt to mud. This way, if your jungle is contaminated, you can clear the corruption/crimson/hallowed with a clemtaminator or purification powder, you can just plant a few jungle grass seed on some mud at the edge of where the jungle was.
I also have always thought the same, and dirt corruptableOr if it was made so that the jungle could be corrupted but it didn't destroy the biome. So instead of going from jungle to corruption/crimson/hollow it went to corrupted jungle, a hollow jungle, or a crimson jungle. That way we would keep the jungle exclusive items and content but the world corruptions could still spread freely.