All modern x86* CPUs are 64bit anyway and if this move means maintainers are freed up a bit, then I think it's worthwhile for the distro as a whole.

Arch isn't really meant to be that one distro you can stick on a PC from the early nineties anyway. There are much more suitable distros for legacy hardware.

And about the downsides of 64bit: I think the vastly improved address space offsets the improved memory use by orders of magnitues. My Desktop has 8 times as much useable RAM as it could have with 32bit - but 32bit datatypes are only double in size.