In most cases, I’d express the same sentiment myself. People perceive any bonus they can attain most of the time as the default state, rather than a bonus, and if they lose it, they perceive that they’re being penalized. And while that’s frustrating, they’re not necessarily wrong. If it’s a bonus, you shouldn’t be able to consistently acquire it.
In this case, however, the problem is that you take an action to acquire a bonus, and lose it when enough other people do the exact same thing. You’re not failing to meet the requirements, other people are taking it away from you, because they’re doing the same thing you are, for the same reasons. If you excluded the first person who fielded a particular defense, that would address that problem, but make the other angle even worse: now everyone else has to avoid using what they know is a good defense team in order to get the bonus, while one person is then rewarded by both having the best defense team and an additional bonus for it.
If there were a wide variety of equally effective defense teams, such an idea might be remotely tenable. As it is, it’s rare to find a team that can even achieve a 20% win rate. But requiring players to be conscious of what other players are doing would still be an enormous distraction from what in many other respects is very much a single-player strategy game.