Shanks overpowering Loki would do nothing for the Yonko agenda. Shanks one-shotting Kid + Killer while Blackbeard extreme/high-diffed Law should have been your first indication that he's built different. He also made GB twerk while Luffy went Uber Eats diff with haki-nerfed Kizaru.
These events revealed Blackbeard and Luffy's current ceiling while Shanks' is still not defined. Even if you think BB/Luffy are weaker than OG Yonko, that performance gap is too large to cover.
Yonko/Admiral are entry-level combatants for the final saga, it seems.