I am not a huge fan of hers nor do I know her that well. But on surface she doesn't seem like a stereotype
victim of abuse. So I tend to guess her reason isn't to erase her past. (plus
@silent1one says about her recent career)
So it's back to issue of royalty. If idols don't get a yen out of continuing DVD sales they have zero incentive for continuing success. It might not be a tangible incentive to delist the old AV sales. But simply watching others make profit and not get a yen or even a thank out of it. If the system allows a zero cost way to delist it, surely more than a few would do it just for psychological reason. Even I can imagine some profit motive for it. If Mihiro is doing v-cinema, would it be a positive effect to delist AV sales on DMM/Amazon? Perhaps... she might want that the only legit way to see her now is to buy her v-films (which she is continuing to receive income for). If you can see her nude and at same time can see her in sex vids (as younger self) the AV might be a strong competition which cuts into her v-film sales. As I analyzed before, this 5 year rule thing is more than just giving the girls more "protection", it's a deep shift of economic power from industry to idols.
Crazy speculation: the setup now is not sustainable. (1) industry might want to crawl back that 5 year delisting power, but it's almost impossible at least for now; (2) industry might have to compensate for lost income due to Mihiro and similar ex-idols delisting, that means current and future idols' income will be squeezed, IMHO this is not good for fans; (3) one way out of it is change the payment formula so that some part of idol income shift to royalty proportional to DVD sales. If ex-idol gets a checque (even if amount is small) monthly or yearly from those 5-10 years old vids she has some financial and psychological incentive to do nothing against DMM/Amazon. Or, if the sales is so low that the ex-idol gets almost nothing (100 yens for half a year) she might feel more like delisting herself, but also the loss to industry is near zero. Of course this is also a long shot, there are many reasons (excuses) this idea create new problems. But IMHO doing nothing is worse.