The Union of European Football Associations (UEFA) could implement a salary cap for players but it would not likely prevent certain teams from dominating their leagues. Players play on a particular team for more reasons than salary, although salary is probably a major motivation. Players also make money from advertising and other sponsorships. Furthermore, even if players were to be subjected to a salary cap, the teams can always find other ways around such regulations.
The teams could provide players with extra benefits and motivations to stay. Some players may even stay for family reasons or other personal reasons. Implementing a salary cap would not work to prevent teams from dominating their leagues.