Its hard for me to pinpoint an exact year-date when the GOP went to hell and basically was taken over by moderates, centrists, and soft-democrats.
The nomination of Goldwater was a brief revolutionary reversal (a counter coup if you will), but the non-conservative wing effectively took over after Reagan's presidency and has continued to pollute the meaning of conservatism ever since. We had some solid conservatives along the way don't get me wrong - Ron Paul, Pat Buchanan, John M. Ashbrook, and Henry Cabot Lodge Jr - but in most respects the Republican Party has been an abject failure in upholding conservative policies of the Old Right: namely non-interventionism, fiscal conservatism, economic nationalism, and anti-globalism.
Their level of diversity and their ability to enact conservative policies are two separate issues. One has no impact on the other.
If you're confused about the differences between the Old and New Right check this link:
https://en.wikipedia.org/wiki/Old_Right_(United_States)
Edited 2/22/2016 03:34:46