Microsoft's desktop supremacy hasn't been down to "it's got a nice, easy to use interface" for at least 20 years. Rather, Microsoft is (or was, in the pre-post-PC world) everywhere because of (a) licensing stitch-ups with hardware vendors and (b) network externalities: get into Corporate IT departments with Office, then people will want (or need) to use the same OS at home, and then you can strong-arm hardware vendor…
Whilst I generally agree with your statement, I don't think it's due to MS having a good interface as opposed to Linux having a not so polished interface. This was a deal breaker for a lot of people I know in the not-so-tech-savvy world. Things have changed a lot in the past few years, but a lot of damage was done by then.