Oda has consistently emphasized that Luffy’s wings - Zoro and Sanji, are not just about power, but trust, loyalty, specific roles and bond.
Wings are more about Luffy's core crew balance.
Loki joining the crew will add another dimension to it and won't replace Sanji.
It didn't happen in last 28 years because we never encountered such strong character earlier who got directly invited by Luffy.
Luffy's invitation is different on multiple aspects
- It had serious tone
- It was given in the build up stage of an arc - missing with yamao, vivi and carrot. No invitation was extended at all to others.
- It made Luffy the first person who indirectly conveyed to Loki that his freedom matters to SHs.
Even if Loki is stronger, strength alone doesn’t replace narrative importance or symbolic roles on which crew is based.
Sanji’s growth isn't purely linear anyway.