This is not the first time AKB does not sell 1 million copies in the first week since the streak started. Both Eien & So Long sold around 900k in first week, and that's with Tomochin Mariko & Yuko in the title track. I am not a Mayuyu oshi & I do not believe she is THE REASON for this single not reaching 1 mill in the first week (if it actually does not reach it but Mayuyu fans are dedicated so who knows.)
But I also understand why some fans will put the blame on her, that comes with being the Top Girl in the 48group. If it succeed Mayuyu will get most of the glory, when it does not she has to take most of the backlash, comes with the job. If this single truly does not sell 1 million this week, Mayuyu will be the only center who has 2 singles(So Long & Kokoro) not reaching 1 mill and that's a lot of ammunition for nay-sayers.