They already have. You can't buy QuickBooks for desktop anymore unless you want Enterprise, the expensive $4k+/year subscription. They dumped the Pro/Pro Plus and moved all those users to QuickBooks online.
And now they've launched Intuit Enterprise Suite in an effort to move the QBE customers into Online. The writing is on the wall there, desktop is going away.
It's also happening in more specialized areas too. I work in waste management/recycling, and this industry was traditionally windows heavy with thick clients on desktops. Even the truck scale software is moving to web interfaces, as are the dispatching and asset management.
OS increasingly doesn't matter for most knowledge work.
Yeah, there are going to be industries that will probably never move, certainly not within a 20 year timeline, but there are a ton that are moving or have moved entirely to SaaS and web apps.
Up front they won't need to do a full rewrite. They'll only need to make it work well enough under Wine.
At a source level, tools like Avalonia's xpf make porting WPF apps to other platforms easier:
Product teams deciding it's easier to ship on + customers having enough linux familiarity (from their other projects).
And the current crop of Microsoft people on the Windows team don't seem to understand building a platform in the way 90/00s Windows teams did.
It's clear MS moved a lot of their smartest people over to work on Azure products.
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.)