There is a large part of the population, as well as a reasonable number of Alton Towers' customers, who don't actually like rollercoasters. Flumes and rapids, whilst they still have drops, are a more gentle ride type that can appeal to a wider audience. And one of the reasons the park doesn't do as well in visitor numbers as it could these days is because they no longer cater very well for these guests who don't like coasters.
Most families are made up of a mix of coaster fans and non-coaster fans, so to justify a family trip to Alton Towers, there needs to be enough attractions to satisfy everyone. And it is fair to say that often the people within a family who are in charge of the purse strings are actually non-coaster fans (because generally people lose interest in coasters as they get older). So if the parents/grandparents in a family no longer perceive that Alton Towers has a good enough range of attractions to keep everyone happy, they will stop attending, especially at times when people are more budget conscious.
Given the recent trend at Alton Towers of removing popular non-coaster rides in favour of coaster rides, the problem with this strategy is that at some point if the park were to remove another 'family fun' style ride like the rapids, we could reach the tipping point where the park could actually lose more customers, than it gains by building a new coaster.