Maybe not replace but both Hearts and Aberdeen have the facilities and the fan base to make a good start at becoming the next big team in Scottish football were Rangers to die. The fans would flock back if Hearts or Aberdeen were challenging for the title/second place. Hearts in 2005/06 when they finished second were very close to having a sell-out week in week out. Given the European money as well, an Edinburgh or a Dundee United or Aberdeen could really flourish in a division that is not dominated by the Glasgow giants.
Because they are doing so well at representing us these days? The game in Scotland needs a major shake-up and this event could be the start for the changes. To answer your question, I think you would be surprised how many would pick the St. Johnstone option. Being a Killie fan I would pick St. Johnstone, for various reasons. Firstly, when playing the Old Firm many fans (the non-OF supporting fans) stay away. This is because of the nature and environment that these games are played in. No-one wants to take their kids to games where "Oooh Aah Up The Ra" or "Up To Our Knees in Fenian Blood" or "Bobby Sands" are being sang. No-one wants to physically and verbally abused at their home matches. I have been spat on, I have been punched, kicked, called a "hun", called a "fenian" these things have only happened at Old Firm vs. Killie games. People can talk about atmosphere and all that the Old Firm fans create, but give me a game against Hearts, Hibs, Dundee United or Aberdeen any day of the week over playing the Old Firm.
Take away Rangers and most of that 45842 will find other clubs to support, be it another SPL side, their local side, junior side. Get rid of the stupid 10 against 2 voting system which allows the Old Firm to veto any changes that are to be made. Allow the SPL to grow in numbers expanding to either 14 or 16. Allow for games such as Dundee vs. Dundee United, Ayr vs. Kilmarnock, Ross County vs. Inverness, Dunfermline vs. Raith to be played in the top division. Games like these would bring back the fans, without the vile chants that are found at Old Firm games.
Call me delusional, but I firmly believe that Scottish football can survive without Rangers.