You're right, the 350-8 was selling very slowly. I believe Hawaiian was the last one to cancel. I seem to remember that Airbus only launched the 330Neo after the 350-8 cancelation but I might be wrong.
From the top of my head, I think the 330neo is capable of 6750nm. I'm sure the 350-8 was way more capable than that since the -9 and -1000 are at around 8200-8300nm.
The market has spoken though.