Great thoughts and points, but I think you're wrong!
Here's why:
First - you make a good point about swapping the toons, but consider the alternative. Would you like to have all on the same toon? Of course you would...hence that suggestion. You, and everybody else. Making it sort of a pain is a good thing, and it really does have an impact. Consider that demand is the same in both scenarios - only so many people want great swords, foodstuffs, etc. Now consider having 4x as much supply....because that's exactly what you'd have when the only barrier to 400 crafting is a small bit of time and money. 4x as many people making greatswords, foodstuffs, etc. 4x as many people fighting over the same supply of materials you are currently pooling from. So more crafters (supply)?
Lower sale price. More people fighting over materials?
Higher cost to craft. Much more people simply crafting something for themselves?
Zero demand.
Obviously "4x", and "Zero" are not accurate, but I'm mainly trying to illustrate that creating some form of annoyance actually has a vast effect on the market. Under the current circumstances...very, very few people will actively craft on 4 characters, and even those people will be slowed down a bit. This simple game mechanic has a lot of impact.
Second thing I'll throw out there that hasn't been mentioned: karma recipes. Leveling crafting isn't enough...as you stated in your post, plenty of people will be 400. Even with level 80 crafts, there will be floods of people who can craft a discoverable recipe.
These margins will be very small. The money-making with high level items will not be about having 400 level crafting. Not at all. It will be about acquiring the expensive ingredients in the most efficient manner possible.
Even with easy Heart quest karma recipes, you can see how we are severely limiting the field here. Only people with four level 80s will have access to all the best recipes...and that's just the basic stuff. What about a karma recipe that many people might not have? How about a very expensive one...that means you completed a huge amount of quests? What about one that you really have to earn...say, by crafting every single other item, or by killing a number of foes in PvP, or by completing all the max titles?
Summary: we definitely need some mechanic in the game that artificially limits the field of people crafting. If we didn't have this, there would be zero margin, and hence, no reason to craft.
Alternate suggestion: I'd propose something that meets both your intention (convenience) while still artificially limiting the field. Maybe still require a second character for crafts 3-4, still require you earn it all, but allow them to "stand in" or something like that. I dunno.