You're underestimating things. Sure one guy can have multiple women, but why slum it with anyone less than a 9/10 when you have literally millions of better looking or wealthier women to choose from? The fact that married couples exist skews the amount of single men available HARD. They aren't left with all the men in japan. They're left with everyone else. The shitty yakuza dudes, whatever sleazy guys knew how to talk their way into a woman's panties but didn't actually plan to do anything with them,whatever otaku have paid a prostitute before, some misc cheating husband, and some decent guys that happened to have a girlfriend before 18.
What motivation is there for a man to provide for 3+ women? It's setting up a japanese society where any female below supermodel tier is going to end up as society's replacement for all those virgin otaku because they just aren't wanted by the opposite sex outside of a pump and dump before 18. Sure 1 guy can get multiple girls pregnant, but is one guy REALLY gonna work his ass off to pay for multiple babies at once to repopulate japan? The age demographics are already at near emergency level in japan. Killing this much of an entire future working population is an unmitigated disaster.
The fact that society hasn't adapted outside of price scalping so far doesn't bode well for the future 18 year old mortality rate.
This basically takes all the troubles women in japan have when it comes to sex (eg A common problem for foreign women is that they have trouble adapting to the fact that you can't just walk into a bar and get laid) and magnifies them ten thousandfold.
It's not giving women more control, it's giving them less while making them a larger part of the population. It's going to be at least 2-3 generations of men picking and choosing at leisure while women are forced to prop up an aging population without the support they were supposed to have.
55% of the male working population is gone when women used to retire at 25. Do you not understand just how massive a hit that is to everything? How massive a shift that is in societal roles and workloads these women are going to have to replace?