I used to think the way I’d try to tackle a challenge like that would be essentially embracing the ChromeOS model by going web-first for everything so you’d avoid accumulating new accidental dependencies on proprietary software, and trying to shift legacy apps to WINE, terminal servers, etc. Obviously you’d need thousands of exceptions but the important part would be removing the inertia which means that new code starts out being non-portable.

Unfortunately, these days the browser engine market is pretty lopsided so that strategy would need to be paired with something like a requirement to test in Gecko and WebKit because I trust Google’s long-term management even less than Microsoft.