I understand your point, and can't say that you're necessarily wrong.
However, there is a lot that has to be done to restore America to its greatness and it core Constitutional and founding principles.
I think having Cruz inside the administration could have great value in helping Trump clarify his vision to "make America great again." Whatever anyone thinks of Dick Cheney, he certainly had a lot of influence on the policies of the BW Bush administration.
I think Cruz could be equally instrumental in influencing President Trump, if it came to that.
Not to mention that it would make Cruz the presumptive nominee at the end of Trump's term.
Cruz would be great in several slots. VP, AG or SCOTUS. But my preference, at least initially is VP, to make use of his expertise in working things through Congress. Of course, he could do that by continuing in the senate, too. Trump will decide where he needs Ted most.
Cruz for Attorney General.
Cruz has already influenced Trump. Where do you got Trump decent policies from? if Trump had any control over his mouth or any real policy knowledge I would think about him.