Universal apps can be sideloaded. [1] There may be a market for a service that skins the process in an end user friendly way. That market is probably larger than just games. An analog might be Github over Git.
Whether Valve should invest in developing technologies to keep Steam relevant is a business decision. This appears to be announcement of Valve's intent. I would not be surprised if paying off the technical debt incurred by Steam does not meet the investment model of Valve's controlling interests. The Windows ecosystem is littered with technically obsolescent cash-cows: e.g. Laplink.
Because Microsoft is primarily a B2B company and makes a lot of enterprise sales, I suspect that from a practical perspective removing it would require an impractically radical shift in its business model and sales channels.
Before Bring Your Own Device, there was bring your work home. Windows is to a meaningful degree a business class product sold to consumers. Removing sideloading from normal Windows would make Windows less attractive as a standard platform for enterprise if people cannot use line of business apps on any old device.
Of course, that doesn't make it irrevocable. But revocation seems unlikely to me.
Whether Valve should invest in developing technologies to keep Steam relevant is a business decision. This appears to be announcement of Valve's intent. I would not be surprised if paying off the technical debt incurred by Steam does not meet the investment model of Valve's controlling interests. The Windows ecosystem is littered with technically obsolescent cash-cows: e.g. Laplink.
[1]:https://msdn.microsoft.com/en-us/windows/uwp/packaging/packa...