Using your own example, the f15E is the best toptier in the game despite being one of the older ones
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Since this is already turning into a classic insult-spam topic I’ll close it and give the answer myself. The reason why some nations might have equipment in game at top tier or at BRs where other nations have more modern stuff is because BRs are not balanced on date of introduction, but rather on the technology implemented in such machinery.
A perfect example is the SPGs that we have in game, most of them are placed at BRs that don’t show how things would have gone down IRL, since they conveniently encounter WW2 tanks. But those tanks are clearly placed there to give them the chance to perform. So whenever you see an aircraft that was from the 90s fight against another one from the 2000s keep in mind that other countries had different periods of development, some of them quite late compared to the others.