You know what? I get it. I get why Sentinel has made it so far.
The Autobot machine exists to serve and reassure itself of its superiority. Ultra Magnus had told OP that he wasn’t hero material… but clearly their definition of hero is different from “person who does what is right even when it’s hard” etc. I think what Magnus wanted to see was an unquestioning, loyal bot… just look at Omega Supreme! Their ideal soldier/weapon is someone who can hardly think for themselves, falls in line, and protects Autobots even at the cost of their life. They talk about “the greater good”, but really they are just talking about preserving the Autobot regime.
Sentinel is a ‘yes man’– always telling UM what he wants to hear, and aggressively doing what he can to preserve the Autobot way of function. He’s an idiot in a lot of ways, and I think UM knows this. There are bigger forces at work here. I think UM lined up Sentinel as his successor because he wanted someone SO DENSE, and SO LOYAL and rooted in the Autobot way that in the event he goes offline he can bet within a degree of certainty that THEY will continue carrying out what he had established. Sentinel hates organics; Sentinel distrusts Decepticons; Sentinel DOES want to keep Autobots alive, and is willing to sacrifice anyone or anything to see that happen… kinda like how UM onlined Omega Supreme, knowing full well that this sentient being would someday pay the ultimate price.
The Magnus doesn’t like to get his hands dirty unless he has to… and this why OP was not 'hero material’. He wholly accepted the blame for what happened with Elita-1– unacceptable behavior for someone who would be in the public eye and a source of morale. OP is too much of a free thinker– he befriended organics and has made compromises with Decepticons to save lives. That’s different; that’s not what Autobots do. The Magnus is supposed to be this impeccable figure… and I’ll bet Ultra had groomed Sentinel to be more of a figureHEAD than a new and independent rule, continuing the plans for society HE had set in motion for generations to come.