At my daughter’s school (in the US) failure to complete CAS means she will not receive the IB diploma. She will still graduate #1 in her class with a 3.99 UW GPA having taken all IB classes her junior and senior year. CAS isn’t actually a class, nor is it on her transcript.
Last year was the first year for IB at her school, of the dozen or so diploma candidates only one person actually received the diploma. I don’t expect many more to receive it this year (due to both CAS and the extended essay).