I was reading about the record football attendances today
Rangers 118,567
Celtic 92,000
Manchester City 84,569
Manchester United 83,260
Chelsea 82,905 (apparently 100,000+ attended a Chelsea v dynamo Moscow game after ww2)
I don't doubt that joining the English league would be great for Celtic and rangers. extra tv revenue alone would be a big help. but you still are not in near the top 4 in the premier league for revenue every year.
The world's 20 biggest football clubs in full (2006/07 revenue figures in brackets)
1: Real Madrid (€351.0)
2: Man. United (€315.2)
3: FC Barcelona (€290.1)
4: Chelsea (€283.0)
5: Arsenal (€263.9)
6: AC Milan (€227.2)
7: Bayern Munich (€223.3)
8: Liverpool (€198.9)
9: Internazionale (€195.0)
10: AS Roma (€157.6)
11: Tottenham (€153.1)
12: Juventus (€145.2)
13: Lyon (€140.6)
14: Newcastle Utd (€129.4)
15: Hamburg SV (€120.4)
16: Schalke 04 (€114.3)
17: Celtic (€111.8)
18: Valencia (€107.6)
19: Marseille (€99.0)
20: Werder Bremen (€97.3)
14-15th is a bit pessimistic, i was thinking around 7th - 8th the first year.
Still the debate of where teams would be placed and crowd trouble would make a joined league rather difficult to pull off.