So build the capability and people will use it when they need it. My point still stands that 100Mbps is slow, even if most people are fine with it day to day.
Also, for a family of four, that would mean only 2 of them would be able to watch a 40Mbps HD stream at once. I get that that is relatively rare for 3 people in a family to want to stream at that speed at the same time, but I wouldn’t call something fast if it can’t support even that.
(YouTube recommends a bitrate of 68Mbps for 4K 60fps content and 45Mbps for 4K 30fps. Higher when using HDR.)