In other words, for the average organization there is virtually no upside.
The only caveat to this is if you do a true intrapreneurship type deal where you are building something within the scope of your employer that they might leverage for operations or as a product sometime in the future. But that would most likely require that they have a clear first right to purchase or that the ownership roles are reversed and you are the minority holder which isn't very ideal for the 30-something.
- Worst case: they get 80% of the work they pay you for, and a chronically depressed employee because the side business isn't working.
- Best case: The side business takes off, they lose an employee, and get a share in a risky startup totally unrelated to their main focus. If they wanted to do that, they probably would have become a VC in the first place! :)
Anyone of any skill of any appreciable amount working a 9-5 has plenty of off hours to build their thing.
My question is: who are the people who could be successful with Friday off that can't be successfull with Saturday, Sunday, and after work? My bet is you're talking about serving a null set.
Take a 20% pay cut in exchange for a four day week.