This argument makes several leaps of logic.
1. Not being worth a certain amount does not mean not being good enough for a certain level. It simply means being over-priced. And that's if moving from Napoli to Chelsea can be considered a "next step" in the first place. Osimhen was priced by Napoli at best-in-class levels. He may not be that good, but concluding, solely based on this, that he's therefore not good enough for a best-in-class team is quite the leap of logic.
2. Even if they truly aren't good enough to move on to the "next step," they're both obviously good enough to make sideways steps or even, at worst, take a moderate step down. So, not being good enough for the next step can't possibly be the reason why they're moving several levels lower. That could only be the case if they've struggled at their current levels. They haven't.
To understand why each player has done this, it would have to be assessed in context.
a. In Osimhen's case, it was a last resort designed to solve a specific problem that arose out of artificial factors: being over-priced and being stubborn.
b. In Toney's case, he preferred the money. These are two vastly different situations that you're conflating to arrive at a conclusion that doesn't even make sense to either case individually.
It's that simple.