Pantheon has raised $29 million to invest more into their WordPress and Drupal hosting platform, with a Series C round, totaling $57 million to date. As is noted in the article, and said to me by a Pantheon employee, they hope this is their last round necessary, which insinuates they are close to long-term sustainable profitability.
And WordPress is a big part of that for them. Though they started Drupal hosting well before WordPress, they tell me about half of their customers are now WordPress-centric, and based on the pace they see, it should increase from here.
They've made progress with agencies looking for hosting tools they can go all-in with (Pantheon's platform is quite thorough, with an in-depth dashboard and an opinionated production-to-local deployment and version control integration).