That's an interesting analogy. If you can find recorded examples of part-time Olympic athletes from a competitive country that would help your point. By a competitive country, I mean one with non-negligible chance of getting a medal in that sport.

I wonder if they exist and if so, how rare.

I suspect it's extremely rare or non-existent for an athlete who holds another full-time job up to the selection time to get selected into a US Olympic swimming team, for example.