It is obvious that smartphones will break the 4GB limit fairly soon (< 5 years), though I think right now the 5S has 1GB.

They added more registers and cleaned up some cruft in the 64-bit ISA, so that is where the speed is coming from. You also get some speed from the extra bits even if you aren't addressing 4+ GB of memory (e.g. 64 bit longs are native).

It doesn't make much sense to clean up the architecture without adding 64-bit at the same time. X86 -> X64 followed the same path.