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This isn't about writing people off. Writing people off is about letting them rot in the gutter, or pushing them back and forth through the prison system. I am genuinely sorry to hear you have lost people close to you to drug-related issues. I do believe that a program like this offers a chance to people who would otherwise not have had it.
Further, by saying the money that funds the injection site could have gone to funding the rehab program, you are presenting a false choice. It's not about funding one or the other- you fund them both, and they work together. The safe injection site functions as a part of the overall rehab program- it's the first stage, the level at which the program makes contact with the addicts. You can include services typically found in a rehab program- counselling, for example- at the site.
Addiction is a complex problem. A safe injection site is not about the rest of society forgetting about the addicts (out of sight, out of mind), or dehumanizing them as you fear, but about restoring a degree of humanity by recognizing that they are like us, and treating them as such.