I guess it depends on your perspective. Is performance a technical challenge? IMHO I say yes.
64-bit enables a number of other things. It has a better ABI for argument passing. It allows more extensive use of tagged pointers (the ObjC runtime can fit many common 11-char or shorter strings in a single tagged pointer). It's also a chance to make any other ABI-breaking changes you want to make in the runtime.
If you're forward looking you might say that mobile devices will exceed the 2GB/4GB limit soon enough so why not make the change now rather than waiting for it to become a problem? You'd get all the performance benefits listed above and a future-proof foundation on which to build.
64-bit enables a number of other things. It has a better ABI for argument passing. It allows more extensive use of tagged pointers (the ObjC runtime can fit many common 11-char or shorter strings in a single tagged pointer). It's also a chance to make any other ABI-breaking changes you want to make in the runtime.
If you're forward looking you might say that mobile devices will exceed the 2GB/4GB limit soon enough so why not make the change now rather than waiting for it to become a problem? You'd get all the performance benefits listed above and a future-proof foundation on which to build.