For most purposes, it has been ready for years. There are a handful of proprietary programs that individual people may need for work that aren't available, and you aren't going to be able to play the latest games on Linux. Most people, however, could switch to one of many Linux distributions and be just as productive if not moreso.
As much as I love Linux (I'm typing this from my IBM Thinkpad running Arch with a KDE desktop) There are far too many warts to call it "ready". Every morning, I plug into my widescreen monitor, and watch as application windows randomly decide which monitor they'll appear on. Then begins the fight with my bluetooth mouse. I enable bluetooth, then turn on the mouse. It's recognized and "connected". But it does not work. I have to disconnect and reconnect. I can't in good conscience advise my mother or wife that this is a "normal" experience, therefore, it's not "ready".
Now, does using Ubuntu cover some of these glaring warts? Perhaps. But that often opens its own set of problems. Each comes with its own set of workarounds. Where macOS and Windows excel, is their sense of "polish". Most happy path things just work, and work 99% of the time.
All that said, I'm die-hard Linux ALL THE WAY! I just couldn't say that it's ready for most purposes. For the things that I'd use a tablet for? Sure.
Then we disagree. Linux works until is doesn't, which is the problem. It isn't a handful of programs so much as a wide range of capabilities. We can all speculate the taste of the average user, but do you think companies wouldn't love running desktop Linux instead of paying millions to Microsoft? That is what everyone did when Linux actually became good enough as a server OS.
I don't see how inertia would be the reason if you also believe that it has been good enough for 15 years. I would say desktop Linux simply doesn't offer enough unique value for the effort involved in large scale deployments, unless you are someone like Google. Most things that have improved for users in recent years, like web application, actually favours Windows. Because the primary use case for desktop operating systems are becoming what is beyond that of web or smart phone applications. Leaving desktop Linux with the lowest common denominator.
I said I've used it for 15 years not that it would've necessarily been a good choice for a non-technical user that long ago. On the other hand, non-technical users usually require significant IT support on Windows as well. That's where inertia comes in. Linux may not offer a significant value proposition over Windows, so Windows stays. That doesn't necessarily mean Windows offers a significant value proposition over Linux either. Inertia.
> That doesn't necessarily mean Windows offers a significant value proposition over Linux either.
But as far as I know that is this case. That Microsoft's offering is much stronger when it comes to large scale corporate deployments. Unless you want to make the claim that e.g. RedHat's offering is on par or better, which isn't something I have heard in the wild.