Your argument refutes itself handedly because 32bit processes are limited to 4GB of address space. You've maxed out 1Gb now, how long before you max out 4Gb? Memory concerns are clearly the reason to go 64bit, the stuff about registers is icing on the cake.

Look at the big picture, whether it be pointers or integers into an array: the size of the index grows with the log of the number of things being indexed. You can't avoid this growth, but it matters less and less as you grow.