It wouldn't offset the price of fast passes - people pay from £35 to £140 for these passes at Alton Towers - and many of these will be family groups, so for a family of four that could be £560 on top of the standard entrance tickets. While the food and tat sold at these parks is overpriced, it's unlikely a family would spend £500+ on it however much free time they had - and it isn't as if no one buys it in the present system.
I am not defending them - the reverse, the whole monetisation of queueing is a disgrace and a racket, and has led to disabled people in particular suffering, but to everyone who visits having to pay to receive a service that shouldn't even have to be a service - getting onto a ride within a reasonable time, say 20 minutes absolute max.
As I said earlier, people need to start voting with their feet - boycott these places until they implement a fair ride allocation system - the technology is there, there's no excuse at all not to offer timed slots to all.
I haven't been to a theme park for about 15 years and I wouldn't go again unless that kind of system was introduced.