You mention waste, and actually that’s another interesting point. It’s no secret that Linux words wonders on older hardware, precisely due to its high level of optimization and low storage space requirement. Therefore, it could be argued that using Linux and other FOSS would quite literally reduce the amount of e-waste produced each year, since people would be able to use the same computer for longer.
I think that’s the big thing. The vast majority of computer users need little more than a bit of word processing, YouTube, maybe some online banking. Beyond that? Nothing at all.
These tasks require such a ridiculously small amount of computing power when compared to other tasks, such as gaming and video editing, that 90% of the power their computer has is just not needed, and is instead being consumed by Windows.