For what it's worth - here's my two yen.......
1)Dream Morning Musume aren't a bunch of old timers- They are in their prime age as performers- 25 to 35 years old,(and Yuko is awesome, so no age jokes, and Koharu is way younger than the rest of DMM) A BOATLOAD of talent. Morning Musume is more like a young team with a couple of great vets, but definitely a team of the future. This was an All Star game, and fans came to see people who they are familiar with, not equal time sharing.
2) of the current Morning Musume, Gaki-san, Reina, and Sayumi are the only ones who really worked with many of DMM (Aika had some time with some of them, but I don't think she is involved with this performance)
3)This is Japan- DMM are the Sempai, and for any of the 9th gen and 10th gen to be even allowed to take part in the performance is a HUGE thrill for them- they are going to be delighted to serve as backdancers
4) I like the new members a lot (9th, 10th, new S/mileage members) but they have been getting their fair share of appearances during the Mobekimasu promotional tours, appearing on regional shows, where the hosts are thrilled to have members of an idol group appear on their show. Gives MM a chance to build up excitement and awareness in the regional markets, and we have girls representing some prefectures that will surely be proud to see a native daughter in the 'big city/bright lights'
5) This was a big show centering around some nostalgia and some of today's headliners. It was not an exclusive Hello Pro event. Producers know that AKB48, DMM, SPEED are going to please this audience, newbies will get their turn when they have earned it. Producers were not obligated to focus on Hello Pro acts.
6) Current Morning Musume are very good, and 9th gen has made incredible strides, and 10th will surely be improving as well. They have a great act, and I enjoy seeing and listenting to them. I think this performance was a great chance for people to be curious about all the new girls, ( I think Risa, Reina, and Sayu are very well known by the general J-pop fans, but the young girls are still a mystery except to MM devotees) and by getting just a brief look at them now, they will say to themselves the next time they see them "Oh yeah those are those young girls we saw at that show, hey they are pretty good!! Wow, great to see them again!!" So yeah, I thought this performance was like planting seeds for a future harvest.