They can easily increase it (CD sales). They still on a relatively low amount HS slots vs Nogi. The key thing is that as long as they don't do what AKB (AKB single taking away potential sales of sister group single) they should be fine. If you can buy a Nogi single with Nogi and Keyaki HS vs Keyaki HS only, which one will you chose? I personally will chose the combined HS because at 1 event I can meet both Nogi and Keyaki members. Also currently Keyaki still have an odd number of members where they don't have enough members to make an effective under group, thus they surely will add 2nd gen member in near future, thus potentially more sales, unless they want to keep this relatively big senbatsu in the future.