It is a good point. But people still can be "looking elsewhere" as in searching
if they can find another job. It is exactly the thing I would do if I had
any chance that I would loose my job. I know my manager would never do it but with layoffs there are greater forces that he may not be able to save me from.
And changing your availability and answering a call or chat from time to time does not cost much. It is not like there is any downside to it.
And if an A player starts looking around great things sometimes happen. He may get hired at much higher position that would justify the salary. Or he may have already earned a lot at FB and may decide this is the time to invest in building a startup, etc.
At my previous company I got in an argument with my boss and just changed my availability on LI. Just out of spite. This led me to an offer that changed my life. And there is a lot of people so much better at their job than I am.