It tomorrow MS and Apple "force" people to pay using their App store, they need some alternative.

MS was a great competitor destroyer. You will compete with them, for example with compilers, like Borland or Lotus or Wordperfect or Netscape and they will destroy you first going against your income generators and they will care about the product later, when there is a monopoly.

MS could destroy steam just using their OS control to make steam perform badly, they did it in the past with companies like DR-DOS, with media encoders-decoders(making them use code that they slowed down on purpose so their badly programmed software looked better) and so on.