I wouldn't buy a half season ticket based on one performance. If Mark Hughes keeps to his guarantee at the beginning of the season and Blackburn look like having an "exciting" season I will certainly purchase a half season ticket without hesitation. In the summer and up to now I haven't been fully convinced (though I don't believe Rovers will be relegation candidates) but winning at Old Trafford was a major confidence booster for me and hopefully others.
Being bored to death for the last 2 years and losing my enthusiasm was the main reason I didn't renew. Instead of getting a full season ticket I decided to go to most home matches (Which I feel is reasonable after the last 2 seasons) and if things have improved by Christmas I will have no hesitation in renewing.