Here's the thing, when it comes to the President the Establishment is much less powerful than when it comes to any other position. The reason the establishment had power for things like Senate but especially House races is that those races require money and most House races especially don't have big enough names attached to draw outside attention. Thus paying works, paying for ads, and all the rest, the Establishment party apparatus is critical, as it's how one gets your name and message out.
But the Presidential seat? There's no need for the Establishment to help with that. The Presidential candidate, simply due to being the Presidential candidate, draws more than enough attention and money to pay for those things WITHOUT the party apparatus to aid them. Yes the party apparatus can still be useful in combing door to door and other such things, but the big power the Establishment has: the power of the purse, doesn't apply to the Presidential candidate.
And further, say what you will about the Republican Establishment and Party, unlike the Democrats they have set the rules of the Primary up such that the Elites have no power to actually sway things. There's none of the Super Delegates who's votes count for more than others, it's a straight up vote by the party members to select their candidate, and that hasn't changed since 2016 AFAIK. The Establishment may not be HAPPY with the results, as they clearly were not in 2016, but you know what, they still ACCEPTED them and haven't tried to change the rules. In other words, if Trump acted as Kingmaker, the BEST the Establishment could try to do is rally support behind a candidate they preferred. And you know what? They have EVERY RIGHT to do that just like Trump would have the right to rally people behind his preference. That's what the Primary is about, the different factions trying to sell their preferred candidate to the party at large, and even though they're often wrong or distasteful, the Establishment moderate types have as much right to champion their preferred candidate as any other person in the party.