People tend to spend an intense few weeks learning a language, then disappear for a year. Then come back and learn another language. A large part of the 2M is still "alive" (I don't know exactly what number that would be, but I'd think around 600k) but Exercism isn't the sort of product you use day-in-day-out.
We're not trying to be some business that sucks people into using our site every day and monetise them. We're aiming to teach people effectively and efficiently and then get them moving on with their lives.
We could make the site less efficient, or change it's purpose, but that's not the mission of the organisation.
If is low all you can do is lower costs, which should also be straightforward.