Immutable data structures tend to use a lot of pointers see how Haskell executes, even in other paradigms halving pointer s can have a significant impact on alignment (tags, enums, etc).

V8 sandboxes WASM by placing them in a dedicated OS process, this buys 64 bit pointers, but comes at a cost. We can chose for a 64 bit processor to buy us in process memory isolation, by masking pointers or we can have 64 bit pointers.