This is such a common idea in any kind of activity that requires any kind of design.
There’s the many time quoted "Make everything as simple as possible, but not simpler." by Albert Einstein.
And at first glance this all makes perfect sense and seems quite wise. The problem is that it doesn’t make for a particularly good thinking tool. It would be great if you could look at a piece of software and go, "ok, let’s make this as simple as possible". This often doesn’t help! It can help for getting rid of some obvious crud, but often this keeps the design the same and simply cleans it up. Finding the simplest possible design out of an unbounded set of designs is a lot more complicated than it first seems.
It can be hard enough generating alternative design ideas, often ideas are constrained by various assumptions, the language your working with, the technology your working with, what you've worked on previously, you knowledge of patterns of design etc etc. How would you feel confident that you found the simplest design?
The more I look into design and see all kinds of alternative ways to design things the more I'm not sure what the exact nature of simple design looks like. Of course there's lots of ideas on how to simplify designs, and there's some great frameworks that make things that were hard to do a lot simpler. But...
I think that there is no absolute simple design. I don’t think it’s even definable given a bunch of constraints (eg, context, purpose, etc)
Not that I think anyone thinks there is an absolute but it is worth keeping in mind that simple design is more about having a "quality" of simplicity rather than being simple or not.