Hundreds of employees. At last count, Valve is around 400 people.You're talking to somebody who has been part of a business. Even a hands off owner has to attend meetings, deal with regulators, make important decisions etc. Owning a multi-billion dollar business, isn't stress free and sitting on your ass, regardless of successful the business is. So no, Gabe doesn't have a stress-free life. He has to make decisions every single day that can affect the livelihoods of 1000s of employees. For example, Steam Deck. There is no telling how many millions of dollars were invested in that. That isn't a chump change decision. Tons of stuff goes on behind the scenes that neither you nor I have any clue about.
I will agree with you on the likihood of MS buying Valve is very low.
Valve is also structured differently from a normal corporation. Very few people outside of the company itself know how exactly it's structured, but from what we know it's a lot flatter, which makes "the man at the top" a far less precarious position to be in.