“Somebody” may have said Reagan couldn’t win, but in 1980 it had only been 4 years since two consecutive Republican POTUS terms.
Nixon won 60% of the vote in 1972. And Ford, a very weak candidate with the Nixon “baggage” only lost to Carter by 3 percentage points.
America was a very different demographic then.
The demographics (based on race, religion, registered voters, party membership, pretty much any way you want to dice it) allowing for population growth, are far more similar to 1980 than they are different.