It can't be only sousenkyo. For example look at NMB48 sales stats over past couple of years:
14/03/26 14/11/05 15/03/31 15/07/15 15/10/07 16/04/27
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
月 ---,--- ---,--- 381,650 ---,--- ---,--- ---,---
火 368,719 324,897 *26,585 282,242 222,115 170,422
水 *15,439 *14,733 *12,531 *19,121 *17,426 *17,659
木 **6,963 **7,496 **7,694 **7,170 **7,657 *27,396
金 **4,973 *62,691 **3,820 *51,566 *48,244 **5,078
土 **4,433 **4,064 **4,444 **3,948 **4,693 **3,533
日 **3,589 **3,434 **6,374 **4,343 **4,425 **3,403
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
計 404,116 417,315 443,098 368,390 304,560 227,491
週 406,579 420,326 447,282 371,276 307,036 ***,***
累 451,949 499,458 530,830 449,148 418,060 ***,***
It's pretty clear trend.