The issue on 1) about extra micromanagement is dead on, it is probably the most significant design cost of such a change. Is it justified? I think so, but this solution may not be the only solution. One could also keep both systems, such that unassigned workshops continue to allow dwarves to produce items of random quality. Now players get the choice to try to fine tune their production if they desire. You could also have a main-toggle to produce ALL items at a given quality, vastly reducing micromanagement (tho in essence the default choice I listed already does this).
So ultimately, the only extra micromanagement would be a player choice. Of course, if this is choice is encouraged (better skill gain if you optimize your selection) then yeah... But again, I argue for reduce resource production, and this means not only items (reducing workplace interactions) but also dwarves (reducing workshops period). Ultimately, I would aim for a large net reduction in management with these changes.
Failure is in there because the game needs some more sinks (resource destruction), and risk in general. Keep in mind, having more sinks means less items, which means improved FPS and less tedium involved with FPS management (which ideally should never be "part" of the game. Also, with increased skill, you would have reduced risk of failure. So if you want that Mona Lisa, practise until you are Leonardo and you will get it.