I agree with you that Trump’s “popularity” with the GOPe is about the same as Reagan’s was with the Bush and happy-to-be-in-the-minority country club Republicans of the late 70’s. But there’s a big difference in the character of the two men. Reagan had an aw shucks, folks demeanor about him that could put voters at ease. And he was a seasoned politician. Donald Trump doesn’t seem to possess an ounce of the kindness and gentlemanly quality that Reagan had. And try to compare Reagan’s ability to deliver a speech with Trump’s stream of consciousness rambling. If he is nominated there will be ankle deep blood on the convention floor in Cleveland and regrouping the party around him will be an order of magnitude harder than it was for Reagan.
Not only does Trump’s bragging and clueless rambling not compare with Reagan’s wit and charm, the two men are operating in vastly different countries.
Demographically, the USA of 1980 no longer exists. White voters now make up a much smaller proportion of the electorate. And since whites are the only real swing voters, the big shift that carried Reagan into the Oval Office is not likely available today. Heck, Reagan would just barely get elected today; and he would be destroyed in his home state of California by any random Democrat.