Could be! Most SaaS tools I can think of cover some specific work flow in a way that it kinda works for 80% of the customers. Their UI is quite an intrinsic part of the offering, an API-only version of it wouldn't be as valuable. But if nobody has to put up with only getting 80% of their needs met, why would they? I'd imagine we'd have a few very general purpose systems that can indeed be glued together in a largely automated fashion. But getting the business processes into them in a meaningful way would then be the hard part, the programming.

I can encourage everyone to read good old "Domain Driven Design", there's a world of interesting problems and work out there beyond the purely technical side of things. And I think that part is a lot more difficult to automate than creating a WordPress site from a .psd.