
I think this is the crux of the matter. To sustain interest, any competition needs to have an element of unpredictability as to who will win. It doesn't matter who specifically challenges for the title just that their
is a genuine competition.
* Nerdy stats alert *
Incidentally, it's not really true that Rangers were pushed particularly hard each season by Motherwell, Aberdeen etc during their 9 in a row - it only really happened twice, in 1990-91 and 93-94. If you take into account that for the first five seasons of their 9 in a row you only got 2 points for a win, then the winning margins are actually quite comfortable. If the 3 points for a win had been in place Rangers would have won the league by 14 points in 88-89, 10 in 89-90, 4 in 90-91, 15 in 91-92 and 15 in 92-93. In 93-94, 3 points for a win was introduced and Rangers won the league by 6 points, then 15 in 94-95. It was closer in 95-96 (4 points) and 96-97 (5 points) but it was Celtic who were doing the pushing (in 95-96 Rangers finished 32 points ahead of the 3rd place team).
Think that proves errr, something, maybe