I don't disagree with you. I think the question is, how much will the Republican party be willing to compromise on (immigration [1], abortion [2], social safety nets [3] [4] [5], drug reform, income and wealth inequality, discrimination) before they're no longer Republicans in order to gain enough votes to remain relevant.
As you said, neither party is going away, but they will both have to change to accommodate an evolving electorate. As someone said upthread, both parties need to stop taking their voters for granted and go out and listen to them.