TBF both teams have massive fan bases, far in excess of most premiership clubs. The issue is the standard of the rest of the league, I’ve watched Celtic on dozens of occasions and honestly the Scottish premier sides are bottom championship to top league one standard at best.
Whilst Celtic and them have huge fan bases they can’t attract seriously good players as the standard is poor, that in turn turns off commercial opportunities! What I would say is that if either ever played in England they would quickly reach the premiership due to the sheer size and resources they could draw upon.
Look at their attendances for “friendlies” at English clubs, support wise absolutely massive, standard wise very very poor - I would expect a relatively even match at Ibrox