As such, I feel their addition can't benefit SKE48 that much.
As opposed to the mass promotions and shuffle of 2009 where AKB48's balance of power changed forever? I understand your point but I think you're blowing it out of proportion. Indeed, new girls being promoted would be a bigger chance of a new star rising, but the odds of them ending up being middle to low tier cannon fodder are very, very high. A Sashihara happens but once in a great while.
From what I heard, the 5th generation is quite strong.
Talent guarantees nothing. Subjective, yes, but I think we can all agree that Sato Mieko should definitely be more than theater leftovers.
I fear that with this new line-up SKE48 will struggle to go to the next level. It will stagnate.
If stagnation is going to happen it barely matters who they promote. The repromoted girls will have just as much or as little to do with SKE's possible downfall as a bunch of girls no one really knows. If SKE stagnates it's more the producer's fault than anyone else's. These girls didn't promote themselves.
And judging by how well SKE did in the last AKB senbatsu sosenkyo, it seems the fandom is pretty happy with SKE's line-up. If even some of SKE's mid-girls get that much support, I think it lessens the possibility of stagnation just a bit more.
Girls like Mitsuki can add some power to the group
Just like how Myao was going to bring some hellfire into AKB. Like above with talent, sometimes it just doesn't matter. The idol world's a bitch. I still contend that these girls will do just as good or bad as any other. I wish I could offer some epic evidence to show that they'll do amazing, but I can't. They're lower-middle girls and they'll probably stay lower-middle girls, likely the same as anyone new until we start seeing some senbatsu girls leave. There's always a chance, though. For any of them. Still, I do see your point that new girls could bring something not-yet-seen to the table. There's no denying that and it is a bummer we won't see that just yet. Something tells me we won't have to wait much longer, though.