One picture is worth a thousand words:
GNU/Linux was doing well until about 2003 according to W3Schools when SCOG threatened the world with law suits if they did not pay them for use of GNU/Linux and that slow-down may have been prolonged by M$’s promise to do a complete re-write to produce a better OS. Vista flopped. SCOG bit the dust and GNU/Linux took off again. OEMs like Dell began to distribute GNU/Linux. The only thing M$ has left in its arsenal is patents on software and those should die soon as even M$ and Walmart and friends are asking the SCOTUS to fix the mess. With Oracle v Google soon to heat up, software patents are dead.
The question remains how far GNU/Linux on the desktop will go. I have no clue, but assuming reasonable parity in quality, MacOS, GNU/Linux and that other OS should get some similar share. GNU/Linux and MacOS have special advantages over that other OS in design but it has the mass at present. It will all work out. We have seen nearly 50% of the share for IE disappear. We could see the same for the desktop.