It doesn't take all the specialized Windows-only line-of-business software being rewritten to have massive defections to Linux happen.

The market you're describing is real, and very significant—but I don't think it's even a majority of Windows users. If so, it's a small one.

And imagine what even 30-40% of all Windows sales disappearing over the course of 2-3 years would do to Microsoft. To Windows as a platform.

Then imagine what would happen if it was 50-70%.

The former, I would describe as "a disaster".

The latter, I would describe as "apocalyptic". (Y'know. For Microsoft as a company. Not in general.)