API management is not big. Is under $500M/year [1] - plus most of the features are becoming commodities. Take a look at the open source KONG API gateway [2]
[1]https://www.forrester.com/Sizing+The+Market+For+API+Manageme...
[1]https://www.forrester.com/Sizing+The+Market+For+API+Manageme...
At a larger size, it may be more expensive than a roll your own, but for an independent idea that you just want to build something as a software/service and if successful be able to scale and handle extra load, it isn't a bad idea.
This is really the missing piece to Lambda in my mind... though being able to act as a bridge for legacy SOAP/WS-* services is pretty nice too. I've written a lot of pretty small node service that act as a bridge to SOAP based services the past few years... though most of those have been for internal use, and not sure how well that translates here.
Also API market is intended to grow to $660 million in US by 2020 and over a billion worldwide (http://blogs.forrester.com/michael_yamnitsky/15-06-07-the_ap...). So it's definetely not small.