There's little to no doubt about it, the lack of/poor promotion was the main factor in their drop in popularity and overall sales of their singles and albums. Now granted, MM and the other H!P groups were still clearly a pretty strong draw at concerts, however, compared to other artists and other idols groups, their sales figures were sad.
One cannot help but wonder how much MORE they could have done and how much MORE success MM could have had during that time (both commercially/financially as well as artistically) had their promotion been able to keep up with the levels of promotion that other idol groups (e.g. the *48 conglomerate, Momoclo, E-girls, BABYMETAL, etc.) were getting.
Bottom line, if you want to be profitable/successful in the music business you cannot act like how TPTB at Up-Front were acting back during MM's Platinum Era. They ignored/denied basic economics and the basic, fundamental premise of one of the biggest factors of what makes musical artists/groups commercially successful; namely, more media exposure leads to more people finding out and potentially becoming fans of the group. The more people you have becoming fans of the group, the more singles/albums/merchandise you sell, which means you're making more money. When you look at the popular idols and idol groups, be it the current ones or ones from the past, what's the one thing that they all have in common?
Their music SELLS. It sells because they're popular, and they in turn are popular because it sells.
The more it sells, the more money you make.
The more money you make = WINNING.
People will not buy your/your group's product if you are not doing a good enough job of:
a) informing the masses that it's actually out there, and
b) making it appealing enough that people decide that they want to buy it.
Make good songs/singles/albums, but more importantly, TELL people about them, show them that it's good and why it's good. Give them a reason to WANT to buy it. Give them a reason to like/love/become fans of your group other than just it's name.