Very true. I was just checking old market share stats (via archive.org) and it turns out that ~20% is what Windows 7 reached after 12 months too. Bear in mind that, while they pushed hard for both Windows Vista and 8 adoption, Microsoft was far more relaxed about leaving people to get Windows 7 in their own sweet time (in fact, adoption remained steady for years) and even backported the improvements down to Vista, so Vista users had little impetus to upgrade at all
So despite having some genuine improvements, being free, having a deadline to encourage rapid adoption, and being pushed so hard it was extremely difficult to avoid, Windows 10 usage is no better after a year than Windows 7 got from just early adopters willing to pay money. In context, 21 market share for Windows 10 after a year is quite dreadful.
And while the first year of sales for Windows 7 was just the tip of the iceburg, with the free period and forced upgrades over Windows 10 adoption is likely to drop off steeply. Of course it will still gain ground, but that will in all likelihood be driven almost entirely by new PC purchases.