Excellent point.
If we take Parksider's comparative attendance stats above at face value, they don't include the loss of support for cup games.
For example, if we compare Parksider's best season of the 1st.Division during the 14 club 1991/92 season with his best season of Super League during the 14 club 2012 season, how do attendances at the cup games compare?
Back in 1991/92 season, there was also the County Cup competitions and the Regal Trophy to consider. These are all games which have been lost along with their walk-up attendances. For example, Wigan's aggregate attendance for the Lancashire Cup in 1991/92 was 30,171, for the Regal Trophy it was 20,356, for the Challenge Cup it was 66,954 (and for the Premiership Trophy it was 30,808 for a grand total of 148,289. In 2012, Wigan's aggregate attendance for the Challenge Cup was 34,004 and 15,467 for a grand total of 49,471. That's a loss of 98,818 fans from just the knock-out cup rounds involving Wigan..
Club - 1991/92 - 2012 - (Loss)
Wigan - 148,289 - 49,471 - (98,818)
St Helens - 120,397 - 26,569 - (93,828)
Leeds - 104,314 - 48,638 - (55,676)
Castleford - 95,155 - 8,130 - (87,025)
Widnes - 65,853 - 9,949 - (55,904)
Bradford - 62,887 - 2,959 - (59,928)
Hull - 51,750 - 28,273 - (23,477)
Warrington - 46,993 - 43,825 - (3,168)
Salford - 35,833 - 6,004 - (29,829)
Wakefield - 33,936 - 10,143 - (23,793)
Hull KR -17,401 - 14,206 - (3,195)
It's also worth noting that Featherstone and Halifax who were both in the First Division in 1991/92 accumulated the following attendances for cup games that year...
Featherstone - 53,051
Halifax - 47,012
Those attendances for cup games back in 1991/92 were primarily walk-up attendances where people paid at the turnstiles and some of them were excellent attendances.
Like you say, today's fans purchase a heavily discounted season ticket for Super League and that's pretty much it, unless their club reaches the final of the Challenge Cup or the Grand Final.