What's wrong with that, though? Of course there's going to be a while where we have to deal with both standards, but that's true of everything. I have a USB A-to-C hub sitting on my desk. It's plugged into my Mac Mini's USB-C port, and all the legacy stuff plugs into it. That's not such a terrible situation.
1) You have to select them (not an easy task when trying to make sure it has all the compatible ports and power requirements)
2) Increases the number of things you can forget to bring that can make your laptop unusable/less useable
3) They detract from the clean Apple aesthetic
4) They increase the cost of your setup, sometimes significantly if you want to match the Apple quality. MacBooks certainly didn't get cheaper when they switched to USB-C only.
I did not have any dongles before Apple moved to USB-C, so I didn't have to deal with any of the above. Also, none of these things needed to happen! Apple could have done what they usually do - gradually transition. Add 1-2 USB-C and then only eliminate the other ports when most folks could do without.
EDIT: One more thing to add - this "transition period" is probably going to last 10+ years for me because I bought a $350 Kinesis Advantage keyboard with a USB Type A connector. It's a solid keyboard that I expect will live at least 10 years.
