There's probably 100 sunken crypts in your world, you're just going to the wrong swamps, as @faceplant8 said. You have to go to larger swamps, because sunken crypts have a parameter requiring they be a certain distance (or farther) from the biome's edge, so in very small swamps, especially the really narrow ones, there's nowhere that they can show up.

In my game the first 3 swamp locations I visited didn't have any crypts. Then i googled and learned about that distance-from-edge requirement, went to a larger swamp and I got enough iron to craft everything, upgrade it entirely, and fill 2 reinforced chests with 'extra' iron out of just that one swamp's crypts.


They really don't need to change anything, there's already a ton of iron out there, you just gotta find it.