Pessimistically, if the next 20 years are anything like the last 20, I would expect most bandwidth growth to come from sloppy development and advertising, plus

Pessimistically, if the next 20 years are anything like the last 20, I would expect most bandwidth growth to come from sloppy development and advertising, plus increased multi-device use, such that even if no single use needs that bandwidth lots of smaller amounts will add up quickly. Several hundred MB to load a normal webpage, before it starts streaming 5Mbps video ad, dozens of IoT devices phoning home orders of magnitude more often than the need to, on top of desired use of 50Mbps 4K TV streaming, and multiple 5Mbps TikTok/YouTube-like streams could easily make 100Mbps seem slow.