Stock market investing isn't a zero sum game anymore than VC investing or even Y Combinator.
Edit: OK, YC adds much more hands on value to businesses, but the investment aspect is exactly the same as it would be if you bought shares over the market.
The reason stock market is considered zero sum is not because it's literally zero sum. It's a well understood and highly competitive market, where most gains come from other players in the market.
Ehhh you get into meta when it comes to options and futures, but I would argue they still aren't zero sum. I mean, you could argue almost everything money wise can be looked at as zero sum.
If a VC buys a 10% stake in company ABC for $1 million the trade looks like:
VC: -$1,000,000, +10% equity (worth $1,000,000)
Net = $0
ABC: +$1,000,000, -10% equity (worth $1,000,000)
Net = $0
Now let's say ABC doubles their valuation. Now the trade looks like:
VC: -$1,000,000, +10% equity (worth $2,000,000)
Net = +$1,000,000
ABC: +$1,000,000, -10% equity (worth $2,000,000)
Net = -$1,000,000
Obviously the $1,000,000 helped ABC double their valuation, which means that the transaction was not zero sum. Same could be said about all investments and to a lesser extent, options and futures. You can't ignore the context of the trades such as using put options to hedge a long position, for example.
Edit: OK, YC adds much more hands on value to businesses, but the investment aspect is exactly the same as it would be if you bought shares over the market.