Paj's comment above about those expecting an overnight, sea change is right on the money.
Glastonbury hosts (arguably) the most famous music festival in the world once a year but you don't very often hear people discussing how amazing the Glastonbury music scene is. The Big Weekend was never going to transform Swindon into a Liverpool, Manchester or London (or even Bristol or Oxford by itself.
With any luck - and over time - it will get to the point where the Oasis is having to turn down big shows due to multiple bookings and then some entity, be it the Council or commercial operator, will have to seriously consider the whole dedicated venue idea.
Beatbullyz have done themselves a world of favours by fulfilling their Big Weekend duties perfectly. Who knows, maybe they'll go on to really have a crack at things - even if not, they got the best opportunity possible and took it with both hands.
Lots of people will have been exposed to bands and venues that they probably wouldn't have been were it not for the BW and associated events. I saw people at both Fringe events I attended that I've never even seen before, let alone in Swindon live music venues. OK, they won't all come back, but some will.
I don't doubt for one minute that the BW will have a legacy... how big that legacy will be, and whether it makes much of a difference, is yet to be seen - but it won't be determined in the same month the BW happened and may not even really make itself fully known this year.