Idk size aside (there are shrinking powers), Loki's way too strong to join the SHs, especially since the SHs are all growing in strength. If Loki grew stronger every arc he'd end up PK level or some shit lol, Sanji would never be able to surpass the guy. Sanji needs to be #3, same reason why Yamato never joined - if her ceiling is Kaido then she's just too strong. If she's not a SH then she can remain uncapped by Sanji.
I see Loki as another Yamato - a super strong candidate they'll leave behind who'll come back for the final war. Could be wrong, but there would never be an M3 again if Loki joined because there's no way Luffy, Zoro and Sanji would become so much stronger than Loki that he wouldn't also be seen as a monster next to them. Again, same reason why Yamato didn't join.
3rd highest SH bounty after Elbaf will probably be Sanji, and if not him then Usopp.