Luffy has been unconscious for the entire week and it’s common knowledge that factions are extremely weakened after large wars. Shanks might have been there for a while. He may have even gotten the Road Poneglyph.
Shanks wasn't there for long time, they had just started talking about Luffy and meeting him and if he was there he should've intercepted Ryokugyu before him beating scabbards and Momonosuke, when Ryokugyu had just arrived
Luffy's been unconscious after all of his fights,
where was Shanks when Luffy was alone in WCI ?
again where was he in Dressrossa ?
where was he when Luffy was imprisoned in Wano ?
and a WEEK has passed, that's ALOT of time for everyone to recover but Shanks shows up after one week and stops Ryokugyu and goes away, again what if Teach comes ?
besides, Luffy has never been more safe than he was now at end of an arc, we are talking about 3xRyokugyu danger in dressrosa when they couldn't even handle one Admiral, while now they arguably can handle all 3 admirals...but Shanks didn't show up there