I dismiss the strength reason coz there are multiple examples that prove it isn’t as black and white as you’re making it out to be.
One counter argument to the strength argument I made is the Blackbeard Pirates who are the foil of the SHs. They have Kuzan who at this point is stronger than all the other members of the crew and he’s unlikely to be either Zoro or Sanji’s opponent. I remember folks arguing that it was impossible for Kuzan to have joined the BB Pirates coz he was ‘too strong’ to as such will most likely take up an ally type role but alas! Kuzan is a certified member of the BB Pirates.
(I was part of that crowd as well btw

)
The second argument is Robin.
When she boarded the ship, she didn’t only have a higher bounty than Zoro & Sanji but was also arguably stronger than both so there is precedent for someone stronger or at the very least someone who had ambiguous standing boarding the ship.
Now I agree with you that when all things are said & done Zoro & Sanji will always be top 2 after Luffy. Same will apply to Zoro & Sanji’s opponents by the time Zoro & Sanji fight them. Hence why Oda insists Shiryu is the No.2 of the BB Pirates even though Kuzan joined.
With regards to Yamato I never for one minute thought she was going to join the SHs and her PL till now is still pretty much still up in the air. Does Yamato beat King for example? I doubt it but I digress. Regardless, if she was ever gonna join the crew, she would’ve. Moreover even on Jinbe, you could argue that his PL in comparison to Zoro & Sanji at the start of Onigashima was ambiguous in the same way Robin was when she joined but things returned to business as usual at the end of the arc.
So yh, the strength argument is not really a strong one for me.
Now if you think Loki’s god tier, one shots Luffy, solos multiple admirals 💀 then I get it lol but I don’t think he is. IMO he’s relative to Luffy at best but that’s just me 🤷🏿♂️