From what I understand,even if you were to were do a post-grad in Mechanical and work in research field,you would still NOT be using 90% of your education.
I’m not sure that this is the right way of thinking about it. It’s hard to know in advance which parts of your education are going to be useful. If each unit of learning only has a 10% chance of being useful, studying 10 units worth of learning rather than just 1 unit gives you much higher chances of at least 1 of those units being what you need. In that case, the “unused” 9 still weren’t wasted, because they increased your odds of knowing something valuable.
Imagine if you only took courses that would help you in the job(lets say the courses where 30-50% of course content(or any other parameter that you choose) directly helps you) and then look at the time req to train.
but this assumes that you know in advance which of the courses are going to help you with your job. If you’ve never taken them and don’t understand their contents, you might not be able to know this.