I haven't seen any usage stats, but I'm not sure the number of developers working on the project is really going to make the higher ups feel good about the move (because it's less than a handful of people actively working on the code).
I think the biggest selling point is that it's open source so even if it goes away tomorrow, your app can still keep on living. Your organization has the ability to maintain the application stack even if development on Lucee stops.
You can't say that with ACF (especially now that they've moved to non preputial licensing). If ACF shuts things down, unless they decide to release a patch that allows it to continue to work for license holders, your app is dead. However, you still won't be able to patch bugs or fix security issues. Not a problem with Lucee (yes, it might take a lot of resources to fix the issue, but still can be fixed).