(1) How will previously illegal drugs be made available? Certainly not over the counter - you will not be able to just walk in and buy powerful street drugs legally at your local pharmacy or supermarket.
They will be available only by prescription.
Says who? Any adult can buy the powerful drug alcohol.
(2) In order to get the prescription, a cottage industry of disreputable medical professionals and social workers will fill the gap. And they will not remain nonviolent
See above.
(3) Whether he gets his fix from a shady prescription mill and its enforcers rather than from a shady street dealer and his enforcers is immaterial to the employability of junkies.
They are unemployable and they will either be subsidized by taxpayers directly, or they will subsidize themselves through violent crime or, more likely, through both.
Or as alkies do: odd jobs, panhandling, can collecting, etc.
(4) Junkies by their nature, need to take doses that are lethal for nonusers and near-lethal for users. The government will not allow such dangerous doses to be dispensed. Therefore, obtaining the desired dose will be illegal in all conceivable circumstances anyway.
Any adult can easily buy several times the lethal dose of the drug alcohol.
(5) Legalization will certainly encourage people who would not otherwise experiment to experiment.
So let 'em - contrary to Reefer Madness mythology, a single experience with a drug is rarely if ever damaging or addicting.
Its status is unique, and meth or crack will not be treated the same way as alcohol in law no matter what form decriminalization takes.
So it's not a good analogy.