Re-implementing "independently" once you have seen the source code can be legally problematic. (see https://www.gnu.org/software/classpath/faq/faq.html#faq3_2 ) for an example.
No longer needing the product kind of means that it's failed. In which case, why would it suddenly be valuable to others with a simple change of license?
It's more likely to succeed in the first place by fostering collaboration from the start. This is how successful open source projects operate. It's all about community, mutual respect, and bi-directional sharing.
Having the source available but not open (yet) strikes me as a cynical attempt to appear open while retaining control of other peoples freedoms, and I for one would never work with such a project. I'd probably rather use a completely closed product with no source available; at least you're getting something that's not pretending to be more open than it really is.