Those examples worked, because you picked out only the working examples.
Has Barca bought players from Madrid? No, (and they probably couldnt if they wanted to).
Have they bought players from Sevilla? Yes.
Would you say that Sevilla never had the potential to become a big team if they kept their players, and it was therefor ok to steal these players and that you werent weakening competition at that point?
Have City or United bought players from Chelsea? No, okay maybe Mata.
Have City or United bought players from Liverpool, Arsenal or Southampton that was a hot club at that point etc? Yes, a motherfucking lot.
Every 5 years or so, Bayern buys 2-3 players from a better-playing rival. Thats basically less or just as much as teams like United, City, Dortmund or even Barca do.
Of course you will tell me that Sevilla doesnt count, because they are no title aspirant. Question is: Would they have been a title aspirant if they had been able to hold their players? I think they'd be some sick squad at this point.