That is the most specious argument Trump supporters make. I was active in that campaign as a volunteer. Reagan was never as personally unpopular as Trump is. In the spring of 1980, Carter was benefiting from a "rally round the flag" effect stemming from the Iranian hostage crisis. But as the year wore on, and Carter showed himself to be ineffective in dealing with that crisis, as well as the issues of inflation, high interest rates, high unemployment, high gas prices, etc., that effect wore off.
There is no correlation between that campaign and this one. People know Hillary is a liar, is a leftist, is incompetent - and they still prefer her to Trump by double digits. That lead is not because she is getting the benefit of people rallying to her side over some international crisis - people don't like her and don't trust her. But they still like her and trust her more than they do Trump.
Your pessimism is very poorly placed. At least wait till the fall campaign is underway to pass judgment. (It is pretty clear that Donald is now going to be our candidate. Give him a chance to demonstrate what he can do, before you write us all off.)