Thank you for the detailed explanation. It makes sense why the ISP can most easily measure the network bit speed, rather than the speed or bandwidth of services carried on the network.
It still isn't a very useful measurement for a layperson customer. In other industries, regulations exist that make it easier for the customer to place the data shown to their real world use of the service. Which in your metaphor might be "how much stuff can I send through the postal service", rather than "how big is the postal truck?." I get that this is a harder question to estimate for, but it is the more useful question to answer for the customer, and questions like these are answered in other industries.
For example, cars are sold with EPA-mandated miles per gallon estimates. Appliances are sold with energy star ratings. They are not perfect estimates, and everything from wind resistance to road quality to driving style is going to affect miles per gallon. The car company might prefer to share the thermal efficiency of the engine, or the rolling resistance of the stock tires, which they have more control over rather than the messy real world miles per gallon. But this isn't very useful to know for the customer, who will be driving in the real world.
If I fire up btop and I run two speedtests at once, I can measure what Mbps I am getting when my connection is maxxed out, and compare that to the speed the ISP quotes. If we convert that to megabytes (or mebibytes if you prefer) per second, and apply a scaling factor to represent the packet overhead, you could arrive at an estimated maximum megabyte/s file transfer speed, which would be more in the ballpark of what a customer would see when they fire up Steam and try to download GTA VI, which is 250 GB.
Packet overhead varies depending on the protocol (I read this for reference: https://linuxvox.com/blog/what-is-the-optimal-size-of-a-udp-packet-for-maximum-throughput/#overhead-when-extra-bytes-eat-into-throughput) but since miles per gallon also varies based on multiple factors, I think giving an estimated max download speed is fair.
I don't see what the downside would be to have the network speed listed in Mbps like always, and also give an estimated file download speed for reference.