Take this ^ theory and apply it to a business where one partner invests cash and the other partner has increased the business due to sales acumen then you'll see how simplistic your argument is.
SS is (thankfully), and should always be IMO, decided upon a case-by-case basis.
In your analogy, both partners are bringing something of value to the business. Sadly, in life, one of the partners is often a useless slug.
I propose an alternative analogy (or story if you will):
Two guys are friends. One guy starts a business, the other one washes the business guy's car every day. The guy who washes cars might be doing a different job, perhaps Tim Horton's, but he doesn't. The car washer isn't very qualified to do much else beyond washing cars. The guy who starts the business is able to focus a bit more on the business because he doesn't have to worry about washing his car. He could probably get anybody to wash the car, and many would do a better job, but his friend is the one washing the car, so he doesn't complain.
Eventually, the car washer decides to stop washing the car, but the business owner still feeds him and clothes him because the business owner is nice like that and still kinda likes his friend. The business owner really does wish though that his friend who used to wash cars would at least do something else that was useful.
One day, the car washer decides to wash somebody else's car, and then tells the business owner that he deserves to get half of the business because he was washing the for the business owner all these years. The car washer invites Arabian over, and Arabian kindly tells him that he really is a wonderful person and he deserves what he wants to get.
Half of the guys in this tale then go on to live happily ever after.
The End.