The post-2016 party is just Trump so, yes, obviously that’s true.
But we we’re talking about the people in the Senate. These aren’t (in majority) people who take up their political positions on the basis of popular fads going around as a background position in their life, like whether to listen to rap or country and build an identity that matches some predefined template.
Most people don’t understand nor truly care about policy. The same people that cheered for Bush to torture terrorists and supported the CIA and NSA through all complaints were quite happy to totally turn on those agencies and any questionable activities once Trump told them that they should. They forgot that the Trans-Pacific Partnership was Bush’s idea, not Obama and Hilary Clinton’s. They forgot that they’d objected to Obama doing nothing about Russia’s takeover of Crimea much past kicking them out of NATO. Most people don’t recall that Russia was a part of NATO.
But the Senators remember all of that. For them, it wasn’t background noise of little consequence past something to chat about as small talk so you sounded like a person who was knowledgeable.
The Senators know Robert Mueller and Chris Wray. They know the generals of the military, the national security implications of things like the President saying that he’d be willing to work with foreign intelligence agencies to win the presidency.
Maybe back, 20, 30, or 40 years ago when they got into all of this, they didn’t have any true or well-grounded opinion on what policy should look like over the long run. But, I’d expect that within a few years that all solidified and has stayed relatively static since that point.
They’re all people who’ve developed an understanding of playing politics - saying one thing and doing another, hyping one scandal to hide another, etc. These sorts of shell games might fool the people but it’s not going to impress your average Senator. Like a magician watching another magician, they’ll see and track every move.
The Republicans in the Senate are, in majority, Republicans of the sort who know what a Republic is and understand that populism is the exact opposite. Making the Republican party the populist party makes as much sense as creating a Communist party founded on the ideas of the Mises Institute.
Unless someone wants to change the name, anti-populists will always push to get under the banner and try to push the populists out. Long term, Trump is taking on a name that is simply too far removed from what it can shoulder.