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That's exactly the issue. I'm just pointing out that it's a fantasy to hope for simple numbering of max supported speeds will simplify the current USB mess.

It will not.

Consumers would expect plugging a 20Gbps device into a 40Gbps port should result in 20Gbps negotiated speed. In reality it will mostly likely end up at 10Gbps (or less) because of the mess.

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Older Thunderbolt devices were not compatible with USB, so plugging them into an USB Type C port would not work.

Newer Thunderbolt/USB 4 devices do not have any technical reason for preventing them to work as USB 3.2 2x2, i.e. to work at 20 Gb/s when plugged into a 20 Gb/s host port, and vice-versa for 20 Gb/s devices plugged into a USB 4/Thunderbolt host port, because both Thunderbolt and 20 Gb/s USB need the same wires in the cable and connector.

I do not know if all USB 4 controllers also work at 20 Gb/s (USB 3.2 2x2), but if they do not work that should be considered a bug.

USB4/TB4 devices doing (only) PCIe tunneling will absolutely not work on a USB3.2 port, or even on an USB4 port without PCIe support (which you can find on some very recent smartphones I believe. It's spec compliant, PCIe is optional.)

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