Pity Thunderbolt 2 is basically non-existent nowadays. I have a few Macbook Pro 13 (2015) and I'd love to be able to use the thunderbolt 2 ports, but peripherals were too expensive and the standard short-lived. Try finding a thunderbolt 2 dock anywhere. Filter out all the false-positives (USB-C docks) and the are maybe 10 total on ebay, and they're stupidly expensive for 6+ year old used devices, most without cables or power supplies. Such a pity because they can really extend the useful life of those laptops.
TB3 is backwards compatible so you can use the Apple TB2 to 3 adapter in conjunction with a TB2 cable to hook up any TB3 device to your MBP 2015. I had the mid-2015 15" MBP and used the adapter to hook up an external GPU. If that can work I'm sure a TB3 dock will.
The cardinal sin of USB IF was releasing the 5gbps then 10gbps USB modes.
It should've been PCIe 2.0 x1 and then 3.0 x1. There was absolutely no reason not to do it: PCIe 2.0 came out in January 2007, USB 3.0 came out in November 2008. PCIe 3.0 followed in November 2010 and 10gbps over the USB C connector didn't appear until August 2014.
What USB4 version 2.0 can only do with a complex tunneling architecture we could get "straight": PCIe 5.0 x1 can do 32gbps which closely matches the 40gbps lane speed defined in USB4 version 2.0 (which again came out years after PCIe 5.0 mind you). It would require two lanes, one for RX one for TX and the other two lanes could carry UHBR20 data for display, for a total of 40gbps. This very closely resembles the 80gbps bus speed of USB4 version 2.0 but the architecture is vastly simpler.
We wouldn't have needed dubious quality separate USB-to-SATA then USB-to-Ethernet etc adapters. External 10GbE would be ubiqutious instead of barely existing and expensive. Similarly, eGPUs would not need to be a niche and DisplayLink simply wouldn't exist because it wouldn't need to exist and the world would be a better place for it. You could just run a very low wattage very simple but real GPU instead. Say, the SM750 is like 2W.
I get what you’re saying but not specifically how you’re imagining the implementation. What do you envision the difference between thunderbolt and usb would be in this case? All complex/bandwidth-intensive applications would be better suited to use PCIe directly, but the problem has always been that for various peripherals this imposes a (small) cost manufacturers would rather not pay and would prefer to have the usb spec abstract over.
There would be no choice, there would be no 5gbps USB mode so there's nothing you can do but use a PCIe chip. It would've brought down the PCIe costs over the many years.