I don't think showing number of subscribers is a good selection of if that subscriber supports the issue. It could just mean they want to see what's going on before they weigh in.
I watch a lot of repos, therefore I'm automatically subscribed to all and new issues, that'd show me as supporting everything.
I'm referring to people who explicitly subscribe to a given issue. It's true that this would not allow people watching a repository to explicitly register their interest but I think the percentage of interested parties who are watching a repository is quite low. (GH could estimate this by checking +1 comments on issues for watchers.)
I think the advantage of using subscribers is simplicity. Adding another feature that is arguably similar to the existing subscribe complicates things.
I watch a lot of repos, therefore I'm automatically subscribed to all and new issues, that'd show me as supporting everything.