I'd argue the iMac was a different situation because there was an immediate upside to using USB peripherals, namely they were much cheaper than Mac proprietary

I'd argue the iMac was a different situation because there was an immediate upside to using USB peripherals, namely they were much cheaper than Mac proprietary stuff. For example HP had a popular inkjet printer, the PC (parallel port) version was $150, while the Mac (serial port) version was $300. The floppy wasn't a huge loss either as most Mac users were using Zip disks. There's no real advantage right now in using TB3/USB-C, because pretty much nothing else uses it. And I'd bet that, in 2020, USB-A, HDMI, DP, and even VGA will still be in common use, so I'll probably be using dongles forever.