adding a new way for people to sign into a site that they can already sign into ...

There's a much larger group of people who can't already sign in, i.e. those with Google accounts who don't have a PM account. Now imagine they could just use their Google login here - would that not make it more inviting for them to do so?

More recent languages have slicker, more user-friendly forums and support sites.

I'd agree that's part of the solution, but sadly I doubt it'd be enough (to change perceptions). Being more recent already gives them a significant head start in looking more relevant/cool, and on top of that, these languages tend to be more actively developed as well.