The argument that it is the English Premier league is redundant because Swansea and Cardiff could be playing in it [potentially] in the future. I think that the Old Firm would doubtless improve the entertainment in the Premier League. Everyone would want to beat them for a start. How could anyone argue that teams will not want to travel to Scotland when players of the calibre of Paulo Maldini [among many others] have described Celtic Park as a special place to play on countless occasions, is also worth considering. Financially, they would definitely benefit; the TV money combined with large fanbase [and don't forget that Celtic are the third best supported club in Britain according to average attendance figures] would more than make up for lost European earnings. If they could survive 5 seasons in the EPL, they would be competing for big names.
But thats the question. I seriously doubt that Celtic and Rangers in their current state could get out of the Championship [arguably one of the most competitive leagues in Europe] on first attempt. Also, i can't see any team in England going for it.