I feel like this is perfect being the enemy of good. So lets say only 80% of their staff can get off Windows and the remaining 20% need to remain on it. That's a great start!

And you can require new custom software to be compatible and guarantee an initial market.

It's a strategic decision and of course it's not financially optimal.

And if in 20 years thered still a few windows computers around in their org that doesn't matter

And a recipe for failure. All 100% of their staff needs to be moved off of Windows at the same time.

A few years ago, IBM tried to move everyone to LibreOffice from M/S Office. It failed, the reason why was top level execs and some others were allowed to stay on M/S Office. As time went on, M/S Windows became a Status Symbol. So people went begging and as time went on exceptions were granted. A few even went so far as to buy their own copy, which was allowed.

After 8 months IBM gave up. If you want things like this to succeed, you must be 100% in.