The difference being, desktop printing is marginally more expensive than workshop printing with incrementally fewer materials choices... while 3d printing is orders of magnitude more expensive than traditional manufacturing with radically fewer materials choices.
And neither of those things will change? "shall always be a prototyping technology"? If that's the case, I agree with you. Again, not an expert or even a hobbyist.
"Prototyping technologies" are those with no economies of scale. If you're injection molding, you get efficiencies when you're making 10, 100, 0r 1000. If you're printing books, you get efficiencies when you're making 10, 100 or 1000. Hell, collating is an efficiency. 3D printing is always a 1-off. Every time you make a new one, it's the same as if you were making it the first time. Just like using an inkjet printer- every time you run it, it's the same as if you were running that photo the first time. WIth "manufacturing" you can use "anything." With 3D printing you can use "soft, melty things." It will never change.
If I could buy a 3D printer for $400 that burned fishing line, I'd do it. I think Candyfab is awesome. Bathsheba Grossman is doing this stuff exactly right - "let's use 3d printing to do the stuff we can't do any other way." But I've also done sandcasting. I've also done lost wax. Hell - my dad made my parents' wedding rings out of 1950s dimes using a coffee can and investment wax. And I'm here to say - the materials you can reasonably work with in 3D printing are a far cry from the materials you want in your life... and the materials you want in your life aren't as hard to work with as most people think.