If you placed all Slack workspaces on a spectrum, at one end there would be the ghost towns, and on the other end, there would be the workspaces with thousands of users that end up being glorified job boards. When you find something in the sweet spot somewhere in the middle, it's truly great.
Part of what makes a great community is having some sort of skin in the game, or even some sort of negative reinforcement for not adding value or participating. I've seen this manifest in a couple different ways. One is to remove inactive members, which I think is a reasonable approach. But it's better if you never have inactive members in the first place, which in theory can be done with a solid interview process and having a culture of giving more than you take.
On some level, a company is a community. These types of communities are active daily. Can you have this active of an online community that doesn't come in the form of a company?