It's not just security. When Opus 5.5 got more capable in September (anybody know why? they definitely did something), we suddenly got flooded with a bunch of old debt that it kept finding and suggesting we fix. And it was right, in a majority of cases. Ate us 2-3 frantic weeks easily just to get from a deluge to a ... well, not yet a trickle, but something more manageable.
Opus 5.5 was released at the end of September, could you have switched from 5 to 5.5 without thinking much and noticed how much better 5.5 is?