Gandalf out ranking and breaking his staff is more of a narrative tool tô help us understand his "upgrade" after being reincarnated on a stronger body.

Saruman, much like sauron and morgoth, put his essence into The things he created. Most notable The uruk-hai, but also his own version of a ring of power. They couldnt tap into a infinite power source like most magic users usually do.

As an example: morgoth, as melkor, was The strongest Vala - valar were "gods" that ruled under eru, The all powerful God of middle earth. Melkor was constantly jealous of Eru creations and ever since The beggining he tried doing his own. His power wasnt enough tô create things per se, so he corrupted,reshaped. This made him weaker as The time went by, because his essence was embbed into his work..

The thing is "magic" in The lord of the rings universe is really limited, and finite. Sauron, gandalf and saruman were higher life forms(maiar) and couldnt do things at will.

Saruman was completetely powerless by the end of the return of the king because he spent all his "magic".

As soon as the "new gandalf" have Access tô unlimited magic you either have sauron and saruman being granted the same prínciple, or the story becomes completely Dull because there wouldnt be a conflict.