I've said it before, and I will say it again, the biggest challenge facing H!P is providing well written songs.
Shirenu san is correct in saying that all of the idol groups have the mix of small percentage of really outstanding singers, and then others who fill in the song.
Nothing wrong with that, the strength of the larger groups (let's say 10 members or more) is found if you can take advantage of actually having a chorus at hand to act as an extra instrument or Orchestra to back the main singer(s).
Trying to come up with catchy songs, when you have to supply multiple groups and soloists is difficult, and that is why so often you hear many Morning Musume songs with the same sound, sort of Euro/sythesizer backing and newish costumes for the PV. Meanwhile Mano chan, S/mileage, C-ute, Berrzy are getting unique songs and PV storylines.All of the groups and solo artists deserve the best songs possible, but there is a pressure to crank out the songs, so that concerts can go on.
H!P is a bit like the Greatful Dead- recordings provide some income, but the lives are where the real income stream is.
Ai is irreplacable, but so was Miki, Maki, Ayaya, and (fill in the blank)__________________. But if you can write good music, and produce interesting PV's, new talent will be able to handle the delivery.