He's not ''stuck'' sealing Ganon, he sacrifices his life to seal him. Meaning that he's still around as a spirit, and we know that spirits can do many things that living people can do(Daphnes uses the Triforce as a spirit in TWW to eradicate Hyrule and Ganon, and Vaati is still able to attack...
Says who?
While we know that there's different characters with the same name throughout the series, this doesn't seem to be the same for sages aside from Impa and Zelda, who were already legacy characters of their own in previous games before. There's only been one known Ruto, Saria, Darunia...
There's more evidence for it being the founding between SS-TMC:
-There's a smoke ring around Death Mountain like in OoT, unlike in BotW or the present day of TotK.
-Rauru is said in Historia to have built the ToT, ending the Era of Chaos and leading to the founding of Hyrule Kingdom, and he...