A GPS receiver isn't really supposed to be a substitute for a paper map, it's more like a substitute for a compass. It helps you work out where you are and/or shows you which way to go. Yes, with a mapping-capable model you can program routes on it directly in extremis, but you wouldn't normally use it that way. It can also act as a more advanced form of bike computer, recording where you've been and how fast you were going when you got there.
A larger-screened, faster-CPUed tablet device with appropriate mapping software compares favourably to paper maps in most respects, at least for touring (where it saves volume and helpfully doubles as an internet access device, ebook reader, etc. etc.). I wouldn't carry one on an audax, and you probably still want the Garmin to keep an eye on things as you're riding along, especially if it's pish.
Paper maps are cheap, the batteries don't run out, and they have large displays. They're susceptible to wind, water, repeated folding, and don't work in the dark or tell you where you are without additional hardware. Updates are expensive.
