I don't think that Saints will need a higher capacity any time soon. I think they have been pretty sensible with the capacity, and is ideal for some of the RFL games.
Clubs should only look to increase their capacity if they sell out pretty much week in week out. Not because 2 games a year could get a bigger crowd, yet otherwise there are thousands of empty spaces.
Wigan are leading the way in marketing and getting the crowds in and offer very reasonable prices, yet even they would only have had 3 sellouts in a ground the size of Saints' this year, and two of those games had plenty of effort in.
18,500 is a good capacity for a Rugby club of either code, and even with 12k in will probably be a very good atmosphere.
I agree that the 18500 is a good capacity to start with and will suit most games; my point is that if Saints can get 17500 to Knowsley Rd for the Wigan and Warrington games, they should, in plush new surroundings be able to sell out 18500 for such fixtures and have demand over that figure if marketed correctly.
It would be a shame if at some point in the future the club was unable to increase capacity in a cost effective manner because such a scenario/ plan hadn't been incorporated into the original build.