Didn't read the Boswell article, but what you say makes sense.
I frankly think pitching is just plain hard to do well on the best of days. You are using your arm to close to the limit of what it's capable of--to the degree that you can only do it every five days to avoid serious injury. On top of that you need to impart various spins to the ball and hit a very small target--smaller than any reputable knife thrower would even consider given the distance (66 feet).
Kelly is a great example fresh in our minds. He actually has a pretty darn good repertoire, especially for a reliever: blazing fastball, curve, slider, and changeup. In other stints this year he has been pretty darn good. But last night once again his 99-100 fastball got hammered (for a solo dinger by Harper to cut the lead to 4-3), and he had to rely on his breaking stuff and did get 2 outs, but was pulled--justifiably--after giving up the walk. In comes super closer Kimbrel, who struggles to get the final 4 outs, the last one on a very hard hit ball that went straight to Benintendi.
Now flash back to the first six innings last night when Porcello had to face that same lineup not once but three times and you begin to understand just how hard it is to be a good starter in MLB and why the best ones get paid so much.