I believe the last Guardian is codeword for the Last Guardian of Console Gaming. It is a prophesy for when the last console game is released, when the console market is on its last legs, it is the guardian of the end. It will be the last console game we will ever get. It will be the end of it all. The end of times. Then we will be in the chaos of PC hell, where there are no rules, no order, no stability, just chaos and fire.
TL;DR: Last Guardian last will be the last good game ever to be released before we reach singularity and are thus killed off by our AI overlords.