Six years ago, I nearly got my ISP to upgrade our fibre connection to 1Gbps. As I said at the time: This is a curmudgeonly post which is going to look ridiculously outdated in a few years. What's the point of Gigabit broadband? Well, it's a few years later and Virgin Media have just given me their Gig1 package for £30 per month. Nice! With all the inflation related price rises, it's great to …
I have made the measurements, and at 500 Mbit/s I actually got a bit more than 5x what I had at 100 Mbit/s. Actually my 500 Mbit connection ran as 550, because the rated speed here is the guaranteed speed of the connection. So the only limitation is the server at the other end.
It is true however that 1 Gbit/s didn’t quite double the 500 Mbit/s speed, Actual measured facts beat speculation.
But your examples of steeply diminishing returns are not true.