Apigee and Mulesoft went "enterprise." AWS API Gateway and GCP Endpoints kind of stagnated. Seems like the only option for small and medium-sized SaaS businesses that want to sell access to APIs is to roll their own or fully commit to RapidAPI. (Please let me know other options I am forgetting or not aware of!)
We currently "roll our own" using Kong, Auth0, and a Rails app. Not sure if we'd want to outsource all of those components. But will keep an eye on this option for the future, particularly for doing metered billing.