People thinking Perospero will help Marco.
Why do you think that, exactly?
When Peros didn't hear about the alliance directly from Big MOm it was one thing, but after that, Marco became his enemy, and the beast pirates his allies.
Do people really think he is going to jeopardize the Yonko alliance in favor of the SH's now?
Please, enlighten me in your thinking proccess, because I don't see that happenning unless Oda completely loses his mind.