The less people use the web, the more they'll depend on the OS. Imagine if Facebook (or Slack) just had to build a native app for every OS they wanted users from, and there was no chance of using anything cross-platform to reach everyone. That would be the sweet utopia for the then Microsoft.

Making the dominant browser shitty is not identical to that scenario, but goes a long way in discouraging an OS-agnostic platform (the Web) in favour of their monopoly OS.