In my experience, moving to GitHub probably is a downgrade from a more sophisticated process in terms of functionality—but it somehow lowers the barrier to entry enough for new people to contribute noticeably more. I'm sure there's some interesting psychology there but I don't know enough to theorize; all I can do is relay my observations.

I don't know if there's some way to "partially" move to GitHub—it would be cool if there were some service that synced GitHub issues with a different tracker, for example, but I've never seen that. Perhaps it would become too noisy.

Of course, more contributions might not be a net gain. For each dedicated contributor, you'll also get drive-by PRs and issues that add noise which maintainers have to deal with. I do not know how that trade-off plays out in practice.

So I can't give a definite conclusion about whether GitHub is worth it, but I would be willing to bet that working on GitHub would get you more contributors.