There's lots of ways to make money sinks. But there's also indirect ways to make 'money sinks' as well.
Right now, the only thing occupying money grinding is I think in fact the Safari Zone. What I mean by this is that there isn't enough things for us to do.
When you are in the Safari Zone, you're not making money.
When you are fully concentrated on breeding to get as many steps as quickly as possible, you're not making money.
When you in some place like the Battle Castle, you are not making money.
Basically, we need more things that allow us to make "progress" or to at least spread the occupancy of our playing time. Unfortunately at the moment, making money endlessly is the only real means of progress for a lot of players right now. Once I have my team be at level 100, I won't have anything left to do but money grind, unless I want to grind for something else specific like lucky eggs or whatever simply for trading revenue.
On the good news, as more features are released, the less people will be grinding money. Like said, breeding. I'll be spending almost if not all of my time breeding Pokemon as quick as I can especially when it does come out, rather than spending hours simply paydaying with EXP share.