You seem to be working under the fundamental misunderstanding that the link is the bottleneck. IT IS NOT. The ISP's router is the bottleneck, because it typically throttles the connection.

For example, my home network has a 100 Mbit bidirectional Ethernet connection between my router and my modem, and my modem has a 152 Mbps downlink (4 bonded DOCSIS channels) and an 81 Mbps uplink (3 bonded DOCSIS channels).

Neither of those is the bottleneck, because my ISP throttles my connection entirely on their router to 3.5 Mbps downstream, 1 Mbps upstream. I, like most in the US, cannot afford a 100+ Mbps home connection.

What you said would hold true IF, say, I had a gigabit connection (but still used a 100 Mbps Ethernet link). Then, yes, upstream traffic would build up in the upstream queue on my own router or modem. But that's not the case, and neither my modem nor router see anywhere near the 100+ Mbps of traffic needed to saturate the link.

If you do not understand this, I suggest looking at the queue depth on an actual router, with all traffic shaping turned off, while running a speedtest. You will see the interface queues in both directions be completely empty.