What's in it for the trader? Life at the bottom of the food-chain probably won't be too enjoyable....
The game has been advertised from the outset as being able to be played alone, with friends or in open play - with the ability to switch between the three modes. If that promise were to be reneged on then I would expect that it would have happened before release.
In the original Elite you started as a trader and although you could pirate or bounty hunt it was completely meaningless even more so than ED, and yet running the gauntlet was quite fun.
Lets just take a look at the pros and cons of Solo mode
Pro's
Traders can make money quicker, (npc's are easy, so you swap shields for cargo space, hence make money quicker)
Con's
Trading in solo thins the player base, breaks the game 'food chain' and could potentially wreak havoc with Pirates having no-one to pirate and bounty hunters having no-one to bounty hunt what happens then ?
All i'm saying here is that IMHO the con's of solo outweigh the pro's to the game as a whole. And apart from traders making money faster really provides no benefit whatsoever and achieves nothing that actually can't be done in open.