I must agree, unfortunately, and do so due to a reason that's way more mundane than "custom formulas": UI.

Language, form, muscle memory (call it what you will) is difficult to separate from thinking and working. I'm very picky when it comes to desktop UI: I use Linux exclusively, and I can't tolerate most Linux distros' default desktop environments. Someone who's been productive for a decade or more with Windows applications -- well, to the extent we're willing to ascribe "UI stability" to those applications' own updates -- will probably hate Linux with a passion.

I don't think such a transition can be made seamless. They should have thought about becoming Microsoft's hostage two decades ago (I guess).

Unfortunately, we have to be willing to make compromises and even learn a new thing or two if we want to survive and protect our sovereignty. And it really is a matter of national survival - Microsoft has made it clear that they are fully controlled by the whims of whoever is in charge of the US government on any given day and will comply with the orders that come down to them. So yes some people will have to re-learn how to use a spreadsheet program, but it's a transition that's worth making.

This is equally an issue migrating from Windows 10 to Windows 11, or desktop Word to Office 365 Word, or in fact basically any major software update.

Yes, there is a cost to changing software. But it’s not unique to an Open Source migration.