[arfar]

Most likely these decisions are not to be made by the developer but rather the employer who actually own the copyright of the code (assuming they're a standard employee). The question from here should actually be:

Can we rely on a proprietary software vendor to push even minor fixes upstream? From the companies perspective, this is giving away the developers time to competitors for free, basically paying for every competitor to be an a level playing field with you. A companies responsibility is to appease shareholders by making money, not to make good software or help software ecosystems.

I'd say that most likely, yes, developers would like to push all their changes back upstream. Also I think that companies/managers not understanding of OSS would definitely NOT want to.