Leicester are from a big City, the surrounding area brings the population beyond 1 million. You can get 40,000 fans every week from that population and therefore sustain a top league side.
But you could argue similar for Bristol, Sheffield, Coventry, Bradford, Cardiff, Nottingham etc. - all cities with clubs that could easily get into the PL and succeed if there wasn't a huge gap to the top. Aston Villa and Leeds are both excluded from the "nine", but it's difficult to argue they aren't traditional big clubs.
Sure, there have always been some more successful, but if you strip out Liverpool's amazing run in the 70s/80s, then prior between the end of the war and the PL being formed there was a remarkable 14 winners in the 32 years that Liverpool didn't win it. Given that Liverpool then went nearly 30 years without a title, the concept of giving up and letting a few top clubs cement the current status is depressing (especially considering the "big six" have already pulled away financially already - do they really need to make that worse?)