I don't think it is unreasonable to assume everyone already has a 64-bit capable PC considering the most recent mainstream 32-bit only CPU is 2004's first gen Prescott Pentium 4.
Intel early centrinos are not 64bit (Dothan / Banias) so are the initial Core / Core Duo CPUs (Yona).
Intel didn't release a mobile 64bit CPU until 2006/7 with Core 2 Duo.
Also the initial implementations of Intel64/EMT64 lack certain functions so even tho they technically support 64bit they might lack certain other features that are required by modern operating systems.
So overall if you have a 10 year old laptop you might not be able to use Tails 3.0.