I believe Rule 12 and Rule 40 invalidates your assertion.
First, Trump was an antiestalishment candidate who won despite those rules - Rule 40 was in effect since 2012, and Rule 12 is purely procedural.. If you're going to claim those rules would prevent a conservative (anti-establishment or otherwise) from doing the same as Trump, you're going to have to explain how Trump managed it, but a conservative can't..
Second, what Rule 40 basically does is boost candidates who win a plurality of convention delegates in primaries and caucuses if you.don't win 8 states, you can't be nominated at the Convention
Why can't a conservative win a lousy 8 states?
Now, if the response to that is "well, there aren't enough conservative primary voters to give a conservative a pluraility", then the problem isn't the system at all. It's that a conservative can't convince enough people to vote for him/her.
I didn't vote for Trump in the primary, and was desperately hoping that a true conservative would catch fire with voters. But it didn't happen. Best guy was Cruz, but the truth is that a lot of people just don't like how he comes across - he was perceived as lacking the personal charm of a guy like Reagan.
But hey, find a conservative candidate who has that charm, and they likely win the nomination. Of course, if the Dems have stacked the electorate with illegals...it won't matter who the GOP nominates.