I don't really care about the powerscaling and I'm not AGAINST Loki joining per se, but it seems Oda does care about keeping old dynamics as they are
People often bring Jinbe but for comparison's sake he was relegated to fighting a tobi roppo while the wings were fighting opponents one tier above (and even get feats against the main villain in Zoro's case) in the arc he joined. Oda made clear where Jinbe standing was.
Loki is out there matching for several chapters someone that dealt with the wings in a matter of seconds . Add to this all the narrative Loki has going on with him (directly acknowledged by Imu as a threat to eliminate alongside Luffy, picked by Ragnir as someone that could enact vengeance on Imu) and it should be easy to acknowledge why Loki is absolutely nothing like any previous non-M3 straw hat in term of treatment lol, the guy is literally the "reincarnation" of a big shot from the ancient era.
Oda subverting expectations and have a guy that can compete with Luffy join the crew as a final crewmate because we're in the endgame would be kind of cool actually, but I'm not betting on it