the explanation is
I'm going to GUESS the average for 3 outs for a side of an inning is 13 pitches thrown... (this is strictly a guess) and avg. 80-100 pitches per starter in a game...
Now my point is - a SP throwing one 30 pitch inning will use up 2+ avg innings of pitching, ergo he will last at most 4-5 innings of work (most likely losing as well

)
so take Shilling or Johnson at thier prime, did they average 13 pitches an inning? probably less but close... 10-11 pitches allowed them to pitch 7-8 innings no problem... now the pitcher with control problems? he might get to the 7th, usually winning because of the offense/defense not pitching.
So my solution is to make stamina and endurance the same with pitch speed and pitch control the variable...
example
stamina - ability to pitch with speed & control 1-5 stars
control - ability to hit catcher target 1-5 stars
Speed - mph of pitch 1-5 stars
a 1 star stamina = 30 pitches/ doesnt mean he cant throw longer - just will take a hit in either speed/control or both
a 2 star stamina = 50 pitches/ same rules apply
3 star stamina =70 pitches/ not good on short rest
4 star stamina = 90 pitches/ avg on short rest
5 star stamina = 110 pitches/good on short rest
Control
1star=70% accuracy/2star=75%/3star=80%/4star=85%/ 5star=90%
Speed
1star=75%/2star=80%/3star=85%/4star=90%/5 star=95% of top posted speed of pitcher - now this variable is harder - a pitcher throwing in the first has all of his gas, a pitcher in the 8th is trying to hang on. I say lose avg 3 % of speed each inning unless hot- then 2% would be good - so a pitcher wont be dominating all game unless he's accurate too.
Just throwing it out there -pick it apart- make it better....