Hmmm, I don't agree. The trend is in the opposite direction. Microsoft Windows used to have a larger market share and supported hardware indefinitely. Now that their market share has shrunk, they are also limiting support for older hardware. This only shows correlation, not causation, but it does show that more competition has not improved the issue and that we need laws to do that instead. MacOS, the primary competitor to Microsoft Windows which also has Microsoft Office available, only supports their hardware for 6-8 years as well.

Edit: just to add, if anything, this actually shows that more competition and reduced market share probably increases the pressure to cut support for older hardware because it probably becomes less profitable to do so.