I have a loved one with an addiction. Calling it that helps us understand why it is a struggle for him to quit instead of just being an easy thing as it is for many others. It gives us insight into the challenges he faces. It gives us compassion for him when he falls, and it helps us know how to pray for him.
Calling it an addiction does NOT make us—or him—say that he shouldn't deal with it, nor does it remove the consequences of his use. If we were to avoid calling it addiction for fear that he might use that as an excuse, that makes it no less and no more of a problem for him.
If people claim that their addiction gives them permission to keep doing what they've been doing ("hey, man, I can't help it"), then let's deal with that. What we call it doesn't matter as much as encouraging people to do what they need to do to get past it.
First, people who want to avoid doing the work will use anything as an excuse. If it isn't the addiction label, it's going to be something else.
Second, while this quoted bit is true in some situations, there's a lot where I have to say "it depends."
Healing takes as much time as it takes. Someone may be seeking the right kinds of support and doing all the hard and painful work, and it may legitimately take them years before that sin is dealt with. Being convicted that what you're doing is sin doesn't necessarily remove the temptations that our crafty enemy has placed right in front of us. Dealing with a pornography addiction/problem/habit/whatever may take years of hard work and lots of sweat and tears.
To me, that is quite different from someone who uses "I have an addiction" as an excuse to take years to get around to even starting the work.
I've seen studies that show how porn is an addiction, and I've seen studies that say that it isn't. I honestly don't care. Whether it's a sinful addiction or "just" a sin, some people have a much harder—and longer—time than others in dealing with it. In that case, chances are that the individual has some deep wounds that need healing, and our energy would be better spent supporting that healing than in worrying about what it's called.