They even had a super compact version. Something like a d400. Was awesome for datacenter/console work. Had a serial port, vga and was like 12.3 inches and only a few pounds despite being stout.
I think I used a d6xx for a while longer than I should have just because of that serial port and how bad usb to serial adapters were back then.
Unfortunately the d420 had a slower processor and would struggle as a desktop replacement.