I think this was at its peak when basically every computer had only USB 2.0 ports. You pretty much could plug in anything to anything, so long as it fit. Not to say there were no issues -- USB 1.1 hubs, cable length limits -- but they were so incredibly minor compared to today.

USB 3 introduced blue plugs, complicating things slightly but still everything basically worked, and if something was operating slower than expected it was at least obvious by the color.

Then 3.1 and USB-C came out, and apparently USB-IF lots its collective mind. Now you have you pay attention to see if your ports, cables and devices are "USB 3.0" or "USB 3.1 gen 1" or "USB 3.2 Gen 1" or "Superspeed" or "Superspeed 5Gbps".. (Btw, trick question: all of those are the same thing).

I don't understand why the color coding plugs wasn't brought back for C. The 3.0 vs 2.0 thing was nice, and made it easy to tell what I needed to plug into. Now I have a phone with a 2.0, a laptop with a 3.1, and a macbook with a thunderbold, and my USB to HDMI port only works with one of the three.

They break aesthetics and for a lot of people that's more important than actual capabilities.

I'm not even sure this was strictly true at one point. As soon as PS/2 keyboard vs mouse plugs, you had problems. Plus you had (and still have) various audio jacks on the computer anyway. Back to the mouse even prior to PS/2 you often had COM1 and COM2 and if I remember correctly you actually could not replug the mouse on the other port in a plug-and-plug way. You also could have very weird video cabling in the 3DFX 1 and 2 era.

But yes, I agree the USB-C plug mess is probably the worst. It has became worse than what it "wanted" to solve. The only way to make it work as broadly advertised again would be if all plugs supported all protocols (that make sense for a given device) and PD options, etc. on all devices and with common cables, at max bandwidth everywhere, but not only I'm not sure this is technically possible, this would probably be cost prohibitive.

And so what is the point? The net number of ports decreases on laptops, and there is no need on most gadget devices (beyond data+PD). The only point is for manufacturers to economize 50 cts and make the new inferior laptops pass as better than the old ones because they use the shinny new USB-C plugs. I would be happy if the price was actually 1/2 of the old models.

Didn't DB-9 also work for early video before VGA?

DE-9 was used for MDA, CGA, EGA, and Hercules. Also some early VGA hardware used it, before DE-15 became sort of a de facto standard. (EGA had separate wires for red, green, and blue, they just carried digital signals. VGA used analog signals for red, green, and blue, so it was sensible to just map them to the same pins. Some monitors could accept either signal format and adapt to the timing of almost anything, the NEC MultiSync and Samsung Syncmaster for two.)

The Tektronics TDS-4xx oscilloscopes produced VGA-signals-on-EGA-pinout, for one example I've encountered in the wild.

DE9 connectors were also used for RS232C serial ports, which included mice and modems and all manner of other things speaking RS232. Wand-style barcode scanners were quite frequently on a DE9 connector, speaking some obscure protocol which may be at RS232C-compatible voltages but probably had positively Martian numbers of data/stop bits, etc. Or maybe just a raw photodiode signal so plugging into a serial port wouldn't do anything useful at all. Heaven help you.

Beyond that they were also used on Token Ring network cards (the hermaphroditic connector being to large to fit on a slot bracket), Fibre Channel in its copper variant (using the same pinout as Token ring, thank goodness they wouldn't blow each other up if mistaken), and pretty much anything else that only needed a few signals. It's a durable, ubiquitous, easy-to-work-with connector.

And of course in industry, the DE9 connector is still used for a zillion more things, like CAN. Pretty much every CAN test tool you'll find uses pins 2 and 7 for CANL and CANH respectively, with 3 and 5 as ground.

Pin 9 has quietly emerged as a sort of de facto phantom power supply, and if you stick 9-to-12-ish volts on it with a polyfuse, a lot of devices will run from that and not need batteries. Some RS232 GPS receivers would accept that, and the Toughbooks with serial ports still produce it.

Here's a funny: RS232 on DE9 is still produced today by the millions, on the backs of fancy TVs, because it's still the most sensible way to interface with high-end home automation systems. (USB never arrived at a good way to do peer-to-peer, and requires entirely too much software.) It's a 60-year-old standard and still has legs.

Monochrome adapters (Hercules) indeed had a DB-9, but it was a different gender than serial (COM) ports. Monitor output had a female DB-9 and COM ports had a male DB-9.