> Comfortable in this context means they no longer have any concern about their own job performance, knowing they have a "sure thing" and underperform in their duties.

There might be multiple things going on here though. While I'm sure there are people who will settle into a job and underperform because they are virtually guaranteed their position, I'm guessing there are a lot more people in a situation where compensation isn't even matching inflation/CoL. In the latter situation the employee may just not care since the company doesn't care enough to compensate them 'fairly'. Depending on a lot of factors it may be difficult for them to find a new job somewhere else, so outwardly it might look like they're just 'comfortable' and underperforming.