I'm actually pretty confident that having the back girls front a song won't affect the sales in any other than maybe back girl wota buying more than usual to show their appreciation.
As for the general Japanese public, they don't know Sayashi from Ayumi from Maachan to be honest. Now that Sayu's gone, probably the only girl they've heard of is... Kanon. In Miyagi, I'm sure Ayumi is very well known, but that's not MM's home base. After Kanon, probably Eripon and Haruna might be the next most well known, but I'm not sure about that. Kanon is the only one I know for sure that non-fan Japanese know of (especially those who are internet savvy since her weight stuff was all over the internet).
What's more important is the songs themselves and the promotion they'll get.
Song wise, I think this is probably the strongest Triple-A single they've ever had. I think both Oh my wish! and Ima Sugu are very striking and have an oomph to them, while Sukatto My Heart is a great song and very Tsunku in a good way.
Promotion wise, it might be too early to tell, but they might capitalize on Zukki's weight loss combined with her shared center position to push it into the limelight. Kanon certainly will if she's given the chance to talk. I'd be very surprised if she didn't make jokes about it and Japanese eat up a dramatic story of people toughing it out, working hard and being triumphant (as any moment of random Japanese television will tell me).
Either way, I know I feel a lot of push to support this myself. I've got a Fortune Music Boxset paid for already, and I'm planning on buying a set for my younger sister in stores since Zukki is her new favourite now that Sayu is gone. I also plan to attend the Selcy Promo Event once I know when it's happening and I'm going to try and drag some people there as well. This truly is my favourite single in a long time, and since I have the ability to I'm putting my money where my mouth is.
Of course.... I wonder if the reason I like this single so much might be because out of all the girls Maachan has received the best treatment
She's lead for one song, has prominant solo lines for another.