People are already talking a bunch about how Yoshida was likely the one to manage to escape Aging's world, right? What if the reward for it was just that, eternal lifespan. And what if Death Devil can't form contracts because the price for it "you get X but die" due to just how powerful she is - so what if Yoshida is the only human capable of forming contract with her due to that?

That could also explain Yoshida's desire to help Denji, and not Pochita - he might be an actual friend of Death Devil, wishing to see her mission complete, and thus wishes to defeat Chainsaw Man, but he does honestly care for Denji, and would rather he could get rid of one without getting rid of the other. This would also give a good reason for why Yoshida would be relevant towards the end of the story - he might be the way through which Death Devil comes to Japan.