We didn't hire Trump to be a nice guy to people who don't produce. We hired him because he is a winner, and knows how to save our floundering country.
Well, unless you belong to the Electoral College, you didn't hire Trump; the majority of America didn't vote for him. And Trump isn't just firing people who don't produce. He's publically humiliating people who don't do exactly what he says when he says it regardless of the laws of physics. Contrary to what Trump may think, McConnell can't vote for other people. Sessions had no way to predict the future so he couldn't have known he would need to recuse himself. Yet Trump is publically deriding them as if everything is their fault instead of doing what most bosses do and call them in for a private meeting about their performance. As they're public figures, maybe some public criticism would be acceptable but not relentlessly tweeting insults. If Repeal and Replace does get done, will Trump put the same effort into complimenting McConnell?
The problem with viewing people as disposable is that you lose the right to loyalty (even though Trump keeps whining about the lack of it). Sooner or later, those you toss away will start to view you as disposable. Go on; try it on your friends and coworkers and see how long it takes you to lose them all.
Another thing to think about. If it's acceptable for Trump to dispose of people the moment it suits his agenda, how do you know he won't do the same with his supporters. He's already broken campaign promises. For example, Palatka Flordia thought Trump would help restore economic opportunities to small communities but he hasn't come through on that promise.
Florida town was hoping on Trump, but now wants to help itself | The Sacramento Bee
You may say doing this takes time, but that's the excuse that Trump is unwilling to accept from the people he criticizes so why should he be allowed to use it?
He can't "jail Hilary" as he said he would because there needs to be an actual investigation. Likewise he has no power to make Mexico pay for the Wall and America is now paying for it.
He's cut Medicaid, although he said he wouldn't.
How can you be sure that, if it benefits him, he won't sacrifice whatever promise you voted for him for?