Very valuable information, thanks for sharing!
I for one am back on buzzerbeater (Canada). I had a team a few years ago and I absolutely loved the experience. I have also invited a friend of mine to give it another shot (he also liked the game as much as I did).
A shrinking user base is normal to an extent. There are so many online games today, most being free or pseudo-free. There is also the problem that the newer generations are looking for instant gratification in their games... meaning a long-term planning game like BB may not be for them. This is a bit sad if you ask me, but that's the way it is.
Eventually I believe the league system could be modified in order to adapt to the more modest user base. Grouping teams by country is great but is it really needed? This system works fine when there is a rather big user base. Otherwise the game feels a bit empty since the active players are all scatered apart. If teams were grouped in a different manner the game would not feel like it is dying. Community would be stronger because more active players who love the game would compete against each other.
Something should also be done to have better user retention. The current tutorial for new managers is great but there needs to be even more hand holding in the beginning to prevent users from feeling overwhelmed and from quiting too soon. In the beginning of your career, there should be a big checklist that appears at each login. The checklist would tell the user the minimum they have to do for the current week. When all the tasks are completed a message could be presented to the user, something encouraging like: "Congratulations! You did the minimum team management tasks this week.". Voila, here is your instant gratification thing, lol.
Enough rambling, I'm just glad to be back.