If we are thinking the same girls that are perma-Under, I think they are graduating or not will not make any difference. I think by now some of those girls are satisfied being in Under as it is. Being Under Member in Nogizaka46 is much better than any underground idols or some dot in a sea of girls called as 48 group. I have some 1st gen Senbatsu girls whom I think should be persuaded for graduation or be put in Unders; and like what you said, free up some space. They are popular enough as they are so being in Unders won't hurt if not it'll do the Unders better. And they're no longer doing interesting stuff that makes them deserve to stay in Senbatsu other than continuing their early glory. Though considering fans don't seem to take graduation quite well, it'll be hard. Look at Fukagawa's case, just how long ago has she graduated and it's not she's like out of the showbis radar, but she is still so long missed.