London has 4X top flight football (soccer) clubs that are known the world over: Arsenal, Chelsea, Tottenham, West Ham.

Paris has Paris St Germain, who currently own two of the most recent top international players on the world stage.

Both cities also have lots of too-tier clubs from other sports, e.g. rugby.

These cities do have plenty of other attractions beside sports though to attract tourists and investment, so without these teams the cities would likely be fine.