You get that it is not unusual for either a Dem or Republican incumbent to get all or all but 1-20 votes, rights? This is not a D or an R thing. It's an incumbent thing at least in the modern era.
What you say accurately describes the GOP convention of 2004 (George W. Bush) and the Democratic Conventions of 1996 (Clinton) and 2012 (Obama). But non-incumbent candidates who fought through the primaries received convention votes in incumbent years (Pat Buchanan in ‘92; Ted Kennedy in ‘80). What struck me about ‘72, and I still remember it, was the lusty boos that erupted when New Mexico had to award one vote to Pete McCloskey under its state law, and the convention didn’t seat that delegate. I can see a parallel for the GOP in 2020 under Trump, but it’s even worse now because of the cancellations of state primaries.
Let's look at since 1948 (Truman was in a weird spot so that is why stopping there):
1948 - D - Truman got 75% but had inherited the Presidency
1952 - No Incumbent
1956 - R - Eisenhower all but maybe 1 vote
1960 - No Incumbent
1964 - D - LBJ all votes
1968 - No Incumbent
1972 - R - Nixon all but 1 vote
1976 - R - Ford/Reagan split but like Truman Ford inherited the Presidency
1980 - D - Carter had Ted challenge
1984 - R - Reagan - Unanimous
1988 - No Incumbent
1992 - R- Bush all but 18 votes
1996 - D - Clinton all but 12 votes
2000 - No Incumbent
2004 - R - Bush all but 1 vote
2008 - No Incumbent
2012 - D - Obama all votes
2016 - No Incumbent
With the exception of inherited Presidencies, there was only one time (Carter administration) where there has been a Primary challenge to the President since the 1950s - getting 20 or fewer delegates is not a real challenge.