Austin left with his body way more intact (except the neck of course) so he's physically in better shape than most nostalgia acts. Hell, a video of him hangin' and bangin' in the gym came out after this potential WM rumor started popping up.
His match with KO showed he was way more capable than we were thinking he would be, and it seems clear now he was using that match to see how he held up after doing it. It appears that he must have felt ok enough that he's down to do another one.
Also, Austin doesn't come off as a guy who would want to keep going over young guys long after he was old and decrepit. Him beating KO didn''t hurt anyone, especially since he overdelivered everyone's expectations.