Read the article. It's not that people can't adapt, it's that they need to make themselves adaptable. Startups need people who can be productive on day one. Someone who has only worked with proprietary technology cannot meet that requirement.
Ergot, if you're working at a place that relies on proprietary technologies, you need to take proactive steps to not tie yourself exclusively to them.
Ergot, if you're working at a place that relies on proprietary technologies, you need to take proactive steps to not tie yourself exclusively to them.