You can't let your competitors come in and steal your best players year after year. Arsenal have done that, Spurs have done that. Both of them put their profits before glory. There are only 3 clubs who are honestly going out to win the league. City, Chelsea and the rags, and if the rags got top 4 on a consistent basis, I think they would settle for that too.
I detest both Arsenal and Spurs, but both of them have the fanbase and rich enough owners to challenge for the title. The problem is, they just aren't willing to invest enough to make a serious challenge.