It depends.

If you feel like you have great product-market fit, and there's a large market of like-minded customers you can dominate: Listen to existing customers and continue to incrementally improve your product.
If you feel like you have great and growing product-market fit, but there isn't a big, profitable market left to conquer (or you've found yourself in a product/market you're not passionate about anymore): Sell to someone who wants to tap that market.
If you feel like you don't have great product-market fit and can't see a path towards finding it: Pivot, hard. Ignore existing customers and focus all your resources on the new direction.