The other was a fairly niche data processing startup in a several billion dollar niche. 10 years in and they missed the boat migrating their front end to the browser and moving their back end off of windows.
Couldn't even sell off the source code in the end.
So it wasn't wasted time in that respect, and they'll be bringing in unique and different skills that they probably wouldn't have developed had they been at MS the whole time.
But...there's things like the lower pay they probably had while doing their business, and the intensity of running a failing business is very very hard mentally and physically.
Near the end, one of the guys shut down...hard...now that he's moved on he's recovering. But I think that kind of experience trails along with you for a long while.
But hey, you're 30. There's time for a comeback. There's no age limit on making a comeback, but I don't think it would be an outrageous claim to say that bouncing back at 30 is easier than bouncing back at 50. Of course, as is the case in most things, attitude matters. A lot. You can look at your 5 year struggle as a massive loss, or as an incomparable learning experience, and that could be the deciding factor in how successful your next venture is.
I would love to run a controlled experiment. Unfortunately we can't force people to do anything, so we never really can eliminate people's choices as a confounding variable.
I can give you one data point from my own experience. We committed to creating a company for two years no matter what happened. After a year and a half, there's no question it was the right choice. If I had chosen to do something else like high frequency trading, programming at a large company, or grad school, the likely outcome would not be on the same level as where we're at now. Even our worst case scenario, selling for a talent acquisition, ends up ahead of those other options.
This is just the gamble that success requires. It's easy to say that objectively they may have been better doing something else in retrospect, but perhaps they felt some satisfaction trying. In any event, it's impossible to know whether someone is going to succeed in advance, or remain unsuccessful.
It appears the decision to persevere, like the decision to "give up", cannot be made rationally. I'd love to be shown otherwise though.