I imagine Luffy right is not strong enough to stalemate or beat Imu, so he is getting saved. Loki is also doubt is lasting much longer once Imu gets serious, if he is indeed final villain like in power.
Shanks wanked too hard for 30 years and inevitable reunion with Luffy sounds likely now to help fight back Imu
Also as far as arcs left, laugh tale could even be very next arc, so when else would they reunite.
There is also a big battle coming between blackbeard, shanks, buggy, Luffy.
Likely 1vs1, not saying a team as a final Yonkou duel and removal of yonkous before crowning the pirate king
Shanks wanked too hard for 30 years and inevitable reunion with Luffy sounds likely now to help fight back Imu
Also as far as arcs left, laugh tale could even be very next arc, so when else would they reunite.
There is also a big battle coming between blackbeard, shanks, buggy, Luffy.
Likely 1vs1, not saying a team as a final Yonkou duel and removal of yonkous before crowning the pirate king