Connections typically don't work well past 80% utilization for one reason or another, so you need a 25% buffer, so 12.5Mb for a sustained 10Mb/s. 5Mb/s average is what's required, you still need to wait for buffering. At this point, your Internet connection is at 100% usage with only two video streams, what about other stuff that's going on? People do like to multi-task. I can't remember the last time I sat and just watched a movie. I usually just stream them in the background and watch them on another monitor between.
The main issue I have with low bandwidth is it has a mental cost that you have to think about what's going on, all the time. What if water was so scare that I had to worry if I'll have enough to drink, if I wash my hands? I hate worrying. Internet is a critical communication tool that is nearly a requirement for anyone working in modern society. It's nearly important as electricity. Whole businesses can shutdown without it, people's ability to work can come to a halt. The internet should work like a light switch, it should just work and I shouldn't have to micromanage who is using it, when they're using it, or how much they're using it. It's not just a personal cost, it's a huge social opportunity cost.