I think you're missing the point. Mobile was kind of the opposite, where instead of having source-available applications on an open platform, the mobile OS makers made it near-impossible to install anything but closed-source apps from their closed-source walled garden app stores.

Being able to run in the browser doesn't mean you have to run it in Chrome. You can use Chromium (open source), or Firefox, or Brave. You can bundle it into an electron app. You could write your own browser that runs javascript with just the APIs needed to make the game work, and deliver it in that (the standards are all open and the spec is public)

This is so much better than what we have with mobile, and much more secure than running random executables on your daily-driver OS directly