When you set up a start-up, you should now give ownership out that generously; give out options that vest over time, and place provisions in the shareholder agreement that specify what happens if one co-founder leaves.
Investors could minimize risk by providing templates for the various jurisdictions that show how this is done, for using lawyers to codify is going to cost more than the startup is initially worth, hence the common but entirely unnecessary infighting.
I use Elementary on one box and like it (only qualm to date: no window minimize button), whereas Ubuntu is nearly everywhere else. It would be nice if the community (= people closer to the founders that they know and trust) could step in and mediate.
It is correct, once you own shares they are yours forever, and unless a contract (shareholder agreement) says otherwise, they are yours to keep. It is NOT appropriate to want to participate in decisions after a full departure. In fact, that may also cause conflicts with one's next employment. Founders that leave may retain a purely passive stake. If they are smart, they understand that that can only be a small stake, otherwise the venture is at risk, because no investor wants much "dead wood" in the share pie.
The broader question is how to monetize a Linux distribution, I find that harder to think about than monetizing open source in general, which is already a little more challenging than selling proprietary software (you can sell services, but while that is easier than selling products, it only scales linearly with headcount => bad idea).
Guys, don't fight over 30k, in tech at your levels that's two monthly salaries!