It does fix the memory fragmentation problem, in that it's almost guaranteed you'll have a contiguous range you can allocate (assuming you have enough memory in the first place). In practice this has been a serious issue.

There is a separate issue of programs that need more than a 32-bit allocation for their memory. Not many apps hit that limit so far, and it's tricky to support anyhow since you need 64-bit ints for pointers and JS doesn't support that well. wasm64 will address this eventually.

It's easier to get a contiguous chunk of memory if you're able to wrangle it in the 64-bit space, then slice out a 4GB chunk for 32-bit code.