Is Trump actually any good, then, at filtering out those who are disloyal to him? If so, then he doesn't have to worry about Cohen flipping. He kept Cohen on as his attorney because Cohen was loyal, having a grasp of Cohen the person. Reading loyalty, like any other attribute, is a skill. Does Trump have that skill?
I'll make an analogy to a basic problem that Trump claims to understand-- investment. Trump claims to be a businessman. He's a con artist, but he claims to know how to make money. Some investment managers outperform the market. Most don't. Most underperform the S&P 500. That means that if you throw darts randomly at a dartboard, with the size of each component proportionate to each company's share of the S&P, and invest that way, you'll beat most investment managers. Yes, really. What about the ones who beat the market? Mostly dumb luck.
Can Trump beat the averages picking out loyal people? Probably not. He picks sycophants who feed his ego. Like... his kids. If that's how he assesses loyalty, then he's in real danger of Cohen flipping. Then again, he may have an actual skill. Has Flynn flipped on him? As of now, we don't know. Manafort is in a separate category given how bad his legal situation is, but he really needs that pardon (hi Scooter!). Papadopoulos wasn't an inner circle guy. Having him flip says nothing about Trump's ability to detect loyalty.
Can Trump assess loyalty? That is the question of the day. His insistence on surrounding himself with family, though, suggests a lack of confidence in his ability to do so.