More that your model was wrong and your math needs to get better.
And for 'discovered' things, there is no reason to make a mistake.
However, most of the time you are working on something that is New.
So I was just thinking about that today. I just bought one of those ridiculous 'incline' rowing machines; as you 'row' it lifts your body off the ground to provide resistance (in addition to the normal rowing motions) - I'd bet money this design lasts rather longer than the type of cheap rowing machine that uses gas pistons to provide the resistance.
There's all sorts of ordinary, everyday stuff that must be engineered to not come apart while using the absolute minimum of material (or the absolute cheapest and lightest of material; I imagine shipping was a significant portion of the cost to me)
I mean, that's the thing about engineering stuff. If you are willing to spend money on lots of high quality raw materials, even someone like me without much engineering background at all can design something that lasts just by massively over-speccing everything. The engineering skill comes in when you have to build a rowing machine that can stand up to vigorous use by a fifteen stone american, but they only give you a buck fifty worth of aluminum to work with.
I mean, I guess what I'm saying is that I look around my world and I see a bunch of stuff that must have taken a huge amount of engineering skill to make (I mean, to make at the price point that I got it at... to be functional in spite of the absolute minimum of raw materials) - would all that stuff be 'new' to an engineer?
If you're not minimizing something, you're not engineering.
If enough people were prepared to pay double to buy something with a 25 year warranty, I'm sure companies would step up and make those products. People don't want to pay enough for it though.
Paying more now for a longer lasting product depends on how much you discount the future (ie effectively your internal interest rate), and on how likely you'll think that you'll even want to use that gadged in the future. Eg the NES was probably ridiculously over-engineered for most people, since realistically they'd only need to not fall apart until you buy a SNES or Playstation.
If I enjoyed selling things more, I probably would have gone for something more expensive anyhow, but I think the light weight is worth a lot in my situation.