MS still firmly monopolizes desktop OS (over 85% market share there). Sure, that market has lost weight due to the rise of mobile, but last I checked we're all still doing our day jobs on desktop computers, and for most office workers (who aren't developers), that's synonymous with working on a Windows machine. For 20-30 years people have had effectively no freedom to choose their OS and there's no change in sight.
Not sure why you're bringing Apple up since they never really were a monopoly. I don't think their very brief ownership of the smartphone and tablet market counts as a monopoly, that's just called being first to market.
I'm not American so I'm less acquainted with Walmart, but a quick Google search shows their stock isn't nosediving either.