While I am not an open-source zealot, more of a open-source novice (using the religious meaning that is), if the Sublime Text project is now under a malaise there is no reason to not open source it.
The arguments above are valid, the time involved to oversee the code pull requests would be a big ask, but is it any more than trying to maintain the project yourself?
Let some other coders in but maintain the last say in what goes into it.
About the whole open-source argument, there is a sort of middle ground that could be taken. There is already a huge number of coders that have developed plugins, and not received any monetary gain for their efforts. Would they not be more than likely to give help to the codebase if necessary? Jon would still maintain the rights but not have as much strain upon him and IF coders wanted to help they could, with no constraints placed on anyone to meet any perceived goals. How could it be anymore slower than it is now?
Also on a lighter note, it isn't the summer of 2014, it is the winter here in Australia, land of SublimeText.