If you bring your car to auto mechanic for an oil change and get it back needing a new transmission because of poor work, would you let him fix it or go to a better mechanic? Why do you think the person that caused the problem is able to fix it?
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Of course the problem with your analogy is that in the scenario you outline

The mechanic totally screwed up replacing an axle, so you brought it back to him -- he then sent it out to get it done right at a discount to what you paid him, and it came back OK

So... when you wanted your engine rebuilt you bring the car back to an incompetent mechanic who this time wants to do it himself

You are worse off then when you started because the competent person bailed out the incompetent boobs