Well that makes two cynics here - I think it will come down to paying for the new stadium ourselves anyway.
The actual capacity will drop overall won't it? It depends whether you believe the Knights' dodgy crowd figures - 19,000 one week looks like a crowd they report as 16,000 the next week - 24,000 is often looking more like 21,000 etc. You can tell when it is full by the amount of space there is:
a) In the Nth-East and South-East corners (over the spectator tunnel entrances)
and
b) around you, i.e. if you have bits of grass between your tarp and the next guy's tarp then it probably isn't that full - perhaps 17,000.
The Knights crowds are easily the most loyal anyway, but we have the crappiest stadium to attend. I think the overal attendance will rise once it is all completed, but sooner or later they'll have to expand it further to accomodate for more crowds.
Anything under 35-40,000 is not thinking ahead in my opinion - 20 years from now we'll be discussing attendances again.