I never said side-projects aren't great for experimenting & broadening your knowledge. In fact, I'm painfully aware of that. It's just that in a startup setting doing side-projects quickly becomes lethal. Maybe us entrepreneurs should get a traditional job once in a while so we can afford side-projects again?
I find that working too many hours outside of 9 to 5 quickly becomes lethal too. And working on side projects as a cofounder feels very much like cheating on a spouse for all the reasons you mentioned.
One approach is to look for future needs and do quick startup related R&D projects in place of true side projects. This doesn't have all the benefits of real side projects though. And it has some real disadvantages too. It still feels like work and can lead to resentment and burnout if you aren't careful. But this the best compromise I've found.
Hasn't been lethal for me. In fact, I'd say that being able to blow off steam in this way has kept me sane and balanced. It may sometimes seem that a startup is a zero-sum game where any time spent elsewhere is time (and runway) lost, but that's a very dangerous mindset to get into. You'll burn out before you even get to the halfway point.
