Penzance can still make sense on a wet day if you switch the plan from beaches and views to galleries, museums, cafes and short sheltered walks. The town has enough indoor options to keep a day worthwhile.
Use the rain to slow the pace down. Start with a gallery or museum, take a proper coffee break and keep one or two sheltered stops in reserve so you are not trying to force a beach day that the weather will not support.
A rainy day also works well as a chance to notice the town itself rather than just the coastline. Some of the best Penzance visits happen when the weather pushes you to stay close to the centre.