I don't think open core is being called into question. By that rationale, anything that is closed source that gets too popular is also called into question. Nothing is stopping AWS or anyone else from copying your API, features or anything else regardless of if your product is open, closed, or some combination in between.

I think if anything is called into question, it's the development of infrastructure software in a startup. Does it make economic sense anymore? As in, is it possible to profit directly off it at this point? Right now it obviously is, but it seems to be trending away from that.