As someone who has spent a lot of time writing and maintaing open source projects, I can say unequivocally that open sourcing ST will not magically help. Open source software only succeeds on a large scale when there is consistent funding behind it. Why? Because someone has to pay for the time it takes even to review pull requests, deal with bug reports, etc. And without some kind of funding, that someone ends up being the repo maintainers. Ask yourself how long you would be willing to put in 5-10 hours per week for free.
Even if the code base is open sourced, how many qualified C++ programmers are there out there in the ST community who are willing to spend lots of their free time over an extended period of time to work on ST? And you would need people who know the native toolkits of OS X, Linux and Windows. This is not a trivial thing that is being proposed. After an initial spurt of enthusiasm, ST is just as likely to be abandoned or progress at a slow pace as open source.