The major premise justifying Windows 10 S, the new variant of Windows 10 that can only install and run applications from the Windows Store, is that by enforcing such a restriction, Windows 10 S can—like iOS and Chrome OS—offer greater robustness and consistency than regular Windows. For example, as Microsoft has recently written, apps from the Windows Store can’t include unwanted malicious software within their installers, eliminating the bundled spyware that has been a regular part of the Windows software ecosystem.
If Windows 10 S can indeed provide much stronger protection against bad actors—both external ones trying to hack and compromise PCs and internal ones, such as schoolkids—then its restrictions represent a reasonable trade-off. The downside is that you can’t run arbitrary Windows software; the upside is that you can’t run arbitrary Windows malware. That might not be the right trade-off for every Windows user, but it’s almost surely the right one for some.
But if that protection is flawed—if the bad guys can somehow circumvent it—then the value of Windows 10 S is substantially undermined. The downside for typical users will remain, as there still won’t be any easy and straightforward way to install and run arbitrary Windows software. But the upside, the protection against malware, will evaporate.