It's not inconceivable for a new coalition to form around a new ideology and have that replace the GOP.
The democratic party after the Civil War was was the political wing of the KKK, in a similar relationship of Sinn Fein to the IRA.
I mean, you could argue that that's happened with Trump anyway; the way the system works, doing it internally to a party is probably easier than completely externally.
As tenuous as it may be, it’s stronger than anyone else’s!
Hasn’t that already happened? Since 2016 there has been frequent reportage that the GOP has increasingly been taken over by populists, that the platform that the party proclaimed before (small government and a libertarian touch, some military hawkishness) has little appeal or momentum today when it comes to deciding elections. When the candidate for president who wanted to run under that party had enough charisma to be elected (or at least defeat all other primary challengers), ideology went out the window. And all without creating a third party.
True replacement would be an entirely new party, from the ground up eventually forming government.
Well, yes, several times. Like I said "in the history of the US..." - because it's happened before.
But what you describe here is shifting platform of the party itself, perhaps due to gerrymandered House districts shifting the rhetoric to include ever more extreme ideals.