Things like algorithmic complexity are well understood and formalized but design patterns are not a science nor has the concepts ever been formalized..
There is no theory or formalized system that says monolithic is better than micro or vice versa, it's all opinion. That's why its' called design.
For your latter point, it’s a matter of elegance, which is a pretty way of saying cognitively manageable. Think of epicycles vs Newtonian mechanics as an analogy. With enough epicycles you can compute the same result, but Newton’s approach is still a clear scientific advance.
Your point about design is well taken. Any given design is analogous to a theory. So we should aim for the simplest and most cognitively manageable design that satisfies our needs. That’s not literally formalized, but it’s a well established principle with an excellent record.
Everyone else, including doctors, who call themselves scientists are just trying to float on the cachet physicists earned with their astonishingly good predictions. Properly speaking, they are phenomenologists . Please note that I'm not saying what they do isn't of great societal and intellectual value! The study and categorization of phenomena is certainly a noble enterprise. But none of them can make predictions good to 9 decimal places.
*might be billionths or quadrillionths by now in QED, but doesn't change my point.