"yes, you are using heroin once a week with clean needles instead of speedball three times a week with shared needles, but you are still using heroin every week!"
Slowing the descent when the alternative is freefall is harm reduction. By definition. What do you think harm reduction means? Going clean cold turkey?
If the "slide towards evil" is "inevitable" then go find a way to fix it, which is easier when things aren't in freefall, and helping stop freefall is lunch on a Tuesday.