The Old Firm joining the English leagues will never happen, and here is why:
1) The SPL would lose almost all television coverage, and the resulting drop in money would crush the other SPL clubs.
2) The other clubs actually like having the Old Firm in the SPL; it gives them somebody to hate.
3) The Old Firm fans are in the unique position of enjoying domestic invincibility, but when it comes to Continental football, they are undoubtedly the underdogs, giving them the best of both worlds. It is doubtful whether either club would win anything in the English leagues, and this would not be acceptable to the fans