I keep forgetting that SV operates under completely different definitions than the rest of the business world. I don't mean offense by that, it's just the truth.
You started a business in 2007 using a 4 day work week and it was acquired in 2008? That's not evidence of sustainability. Smart? Yes, apparently. But is that business still using a 4 day week?
I'd say that's pretty good proof that it's not just sustainable, it's also very smart.