We were Big Six but the Big Six isn't really a huge thing anymore when you have the likes of Villa finishing above teams like Chelsea or Newcastle above Man U. It's more fluid now than it used to be. In the 2010s though it was certainly a thing, and it kinda still is a thing when you talk about their global brands and stuff. Spurs have a more global fanbase than Villa despite Villa being consistently higher on the ladder.
Clubs rise and fall too. In the old school era clubs like Everton and Leeds challenged for or won silverware, back when of course Socceroos greats played for both sides (obviously Tim Cahill for Everton, Harry Kewell and Mark Viduka for Leeds). Nowadays Everton are a midtable club and Leeds only just came back up (and the last time they came up a few . Blackburn and Leicester have both won the Prem, but now are battling relegation in the Championship. Bournemouth and Brighton used to be in League One, while Oldham and Wigan were in the Prem. The list goes on. Sometimes it's fast too, like Luton going down twice in a row and now being merely pushing for playoffs in League One.