1. That doesn't prove him wrong, in any way the presence of Bane, Kaan, Kas'im, Githany, Kopekz, Qordis, Krux Kral or Sirak in the BoD proves him wrong. You can still have powerful people with a limited fuel supply, just like an incredibly powerful darkside doesn't suck simply because he's on a lightside nexus, but it is a valid disadvantage. Bane's opinion is verified by the propechy of the Sith'ari.
2. It's the kind of scenario you would have in a war brosepoh.
3. Hahaha, please. 😂 Sith are genuinely known for betraying each other, Jedi on the other hand are not known for letting their emotions interfere with their duty. And even if it did, Jedi typically become enraged and far more powerful in those scenarios. Anonymous to us, not to eachother.
4. There will still be the opportunity for trachery etc. and a real Sith takes every opportunity they can get to rise above their peers.