TBH, when I first heard "failure to thrive" I thought of a child on life support or worse, and when I was first told that my niece was a FTT baby, that was what was going through my mind and it terrified me, I thought she was about to die. Then I found out what it actually meant.
I think FTT is a very cruel term and scare tactics are not useful when you are dealing with a child, especially an infant. I know how bad it made my sister feel and I don't even want to think about how bad I would have felt if I had been told my DD (who is currently my only child) was a FTT.

So I agree, the term FTT MUST GO!
I like Weight Gain Resistant. I don't like SBS because as mentioned, it's too close to Shaken Baby Syndrome.