Linux and open source removes the capacity for people to compete against it commercially. It takes air out of the room. Why would we pay you when we can get this for free?
free as in 0 cost is very difficult to compete against unless you are 100, 1000x times better.
There is the value that would have been provided (generated) in revenue to UNIX manufacturers had Linux not existed and the value that exists with Linux being free. These are two numbers.
One is a tangible financial number, the other is another tangible financial number from profits not needing to pay for a UNIX manufacturer.
It's difficult to say what would have happened, but you cannot deny that prices for corporate UNIX DID in fact exist, and they were not getting paid due to the availability of the free option.
Also, remember that the UNIX manufactuerers would have produced value too, so you need to include the value produced by UNIX versus the value produced by Linux minus licence costs.
If you're using the argument that if everything was free then there would be infinitely more value than things being commercial, then why isn't everything free, if that truly produces more value?
I would expect everything to be free if that was truly the better option.