MOST relationships in life are disposable, no?
It depends on how one defines "relationship."
If you categorize any interaction with another person, regardless of level of seriousness, as a relationship (for example, you have a relationship with your mom, but you also have a relationship with the janitor because you exchange pleasantries once a week), then YES, most relationships in life are disposable.
My definition of relationship is different. If I consider an interaction to be disposable, then it's not a relationship. I know everyone on my building's cleaning crew, most of them by name, and I exchange pleasantries, but it's not a relationship. I have lots of acquaintances, and many of them are "friends" on Facebook, but I don't consider then relationships. All of these people could vanish from my life without a trace, and it would make no difference to me.
I have relationships with my family, my friends, (some) colleagues--people whom I engage with in a deeper, more meaningful way. Most of them are positive, some of them are negative. If they were to become absent from my life, I would be affected in some way.
Therefore, NO, I do not think that most relationships in life are disposable. Some are not meant to last a lifetime; they each have their own season, but it doesn't mean that they are disposable. Because PEOPLE are not disposable.