You’re of course certainly right about the total whole-path bandwidth in internet acces, as there might be insufficient backhaul or bottlenecks further upstream. I myself don’t experience such limitations and my ISP, AA has a publicised policy of ‘we will not be the bottleneck’, adding capacity so as to keep this true. I was imprecise and I should have said that my DSL link to the exchange, which is the obvious extremely serious bottleneck, is not shared,. And my own maximum traffic rates are so low in any case that the link within BT further upstream is not remotely likely to ever be an issue. Also I pay an extra £12 for traffic prioritisation within BT and AA anyway.
Psychology: I don’t like the idea much of the bottleneck link in an internet access path being shared, not ring fenced. But that’s just me. It’s the uncertainty, even if practically a service were fast most or all of the time but was such that there was a possibility that things could change and it could go very bad due to a rogue user. Service providers can always artificially ringfence bandwidth of course, in a number of very difference ways. It just means that they would have to charge a lot, or even way too much for the majority of users, who do not care about performance, just cost.
I’ve never ever experienced access link performance droop / wilt at peak times, not even in the bad old days.