In the 1990's Lawrence Ricci was whacked by the Genovese family for refusing to take a pleas deal.

I believe the newer generation who had at the time taken over for the previous heads of the families just witnessed their predecessor's get sentenced to in some cases to 50-100+ years.
This was also after Gene Gotti and guys in his crew got hit with 50 years after turning down please for about 10 years.

Based on original Omerta all of these guys broke the code and could have been killed for it:

1) Carmine Persico.
2) Joseph Bonanno.
3) Vincent the Chin.

Every boss of the five families today at one point took a plea and they too would be part of that list.