What does that even mean? Almost every single Falcon 9 customer will prefer launching on Starship if/when it is available, because the cost will be much lower. A very small segment who have payloads that are exactly Falcon 9 sized and want a very particular orbit might still be better served by F9, but maybe not.

Beyond that, much lower cost unlocks previously untenable opportunities that you have not sufficiently imagined, as stated earlier.

Of course it does. With Starship, SpaceX could've charged NASA/ESA more to launch a bigger JWST than the cost to launch with Ariane 5, with huge profit margins.

On top of that, with a much larger fairing, you could almost certainly simplify the telescope and increase capability. A significant part of the JWST's complexity is the unfolding sequence, which could be simplified with a fairing that is more than double (triple? quadruple?) the volume.