^ The problem with the Platinum era Momusu was, I think, poor management decisions. If I'm not mistaken there was a huge change in UF's big wigs' chairs some years ago, a change that has taken a while to consolidate at all levels (big companies tend to be slow in that sense). So I think that this new business point of view has finally paid off: better promotions (get back to the Shibuya posters and TV ads) a great social media strategy, a change in the choreography team, even the MVs have been evolving gradually and the CD covers keep being quite a hit or miss matter but they've been getting good at it too.
In short, it's the management that has learned to read the new tendencies and to make good use of the new media conventions (just compare the HUGE amount of material released on youtube by UF, even super-old MVs, to the scarce material - if any - allowed by AKB48's agency).
That said, perhaps because of their age, these new generations are better atuned to the kind of dynamics that are required now. I don't know if the addition of a new girl would either hinder or help the group at this stage but they might not want to take unnecesary risks.
A new girl should've been like the new Goto Maki(*) to add something valuable to the group
(*)an incredible amount of talent, great poise, unique looks, huge charisma and the ability to automatically blend into the group so that she doesn't stick out like "the new one".
also maybe sakura's the mirai shoujo
*COUGH* Sayashi Riho *COUGH*
As much as I love Sakura's voice (and I really, really love it
) she isn't as well rounded as Maki was. Riho... well, ambition and a crowd of devoted wotas isn't enough either