What's the ongoing work? Don't you post a design and then keep getting the revenue, or do you have to continue promoting it and making new designs to stay relevant?
Eventually, the themes stop selling, unless you've created some sort of product that sells like hotcakes all the time (there are a few). But, most of the themes sell for a few weeks/months then you have to start working on a brand new product again, to keep up with the competition, and to promote your previously released products.
I had issues scaling the business since I was not able to cope with the amount of work that needed to be done (and also because I couldn't find anybody competent enough to provide support / fixes). That is, provide support on the forums, bug fixes, stability & compatibility updates, and releasing new themes. Not to mention having 0% time to work on side projects.
It's not a fire and forget scheme, each product has its own ongoing support service that has to be provided. There are people that purchase the theme just because of your support, not mainly because it looks good enough. For a single person (doing everything) becomes quite a lot to handle, even if the profits are quite lucrative.
If all you ever want to do in your life is sell themes, then the marketplace is great for you. I'm aiming at a more self-managed project, such as a SaaS product, or my own startup, not planning to live out of the themes business, which was one of the reasons why I've decided to stop.