I don't have any strong opinion about it, but I have the same question: is it possible that in the long term the backend will be replaced by a custom one? And if not, what is the postgres backend bringing to the table that's difficult to replace? Sure there will be some friction between edge's model and the way it has to be internally expressed in postgres?
Edit: oh, a relevant reply https://news.ycombinator.com/item?id=30293064