I don't believe it's crucial to opensource ST. I believe Textmate is closed source but look at its success. Jon is providing us an API and I think that's enough.
I'm afraid that it's too hard to beat TM, Vim, Emacs or Eclipse(Aptana...). Too hard for one developer. Yes, we can create a some packages using API, but lets face it: most of current plugins for ST 1 are broken, abandoned, not supported anymore. I'm talking with some rich developers about alternatives and Sublime, but they said: "no, thx, community will come only if sources will available, otherwise - it will die".
Opening of sources - great step, which can boost development and author popularity. But it's harder, of cause.
Just don't compare this case with TextMate or MSVS case. TextMate is older and when it come, it have no alternatives(vim? emacs? no, thx). Now it has great amount of plugins, functionality and fans. For beating this editors and most IDE's, we need to "boost" development with powers of community, but not only in plugins part of dev.