So your argument is don't use an off the shelf tool (5 stages) that gets the job done, build your own tool (10 or 3 or none) every time which likely doesn't offer any advantage over the standard tool?

I don't think you really get either Design Thinking or Systems Thinking.

What you are talking about here is not Systems Thinking, which is a particular approach to understanding complex problems by viewing everything as systems of systems. Design Thinking is a methodology for approaching the design process, which is quite orthogonal to whether or not you employ Systems Thinking. The more general field of trying to understand how we determine whether something is true and what it means for it to be true is the field of epistemology; "epistemological based theory" is a meaningless description, like "philosophical based worldview".