This community has tough rules and like in most history slavery, slaves tend to come and go (or at least most dominants think so).
Are they expendable?
YES, they are!
Every slave knows it’s position and the senior ones even know they can be left behind and that is why most of them “behave” (above the surface).
But what about the moral question: Should they be left alone?
Modern slavery is different in so many wide aspects than ancient slavery. We have rules that are accepted by both of side, we bridge our problems not only by force, a slave knows it’s a slave, we don’t kidnap our slaves and just use them (at least we agree on that first), we do not kill, etc.
In addition to all of that, there is the term of “aftercare”.
Aftercare means that at the end of every harsh session a Dominant should give a None-Dominant the place to lay down, calm and take care of the fear/problems.
It looks weird but we do take care of our problems, those we create during the session, those that a slave bring with and even those he doesn’t tell us about.
The younger Masters have less patience and the veteran ones have more of it but still, some slaves left behind.

Personally – I do not believe in this method, Just as parents do not abandon their children than we don’t abandon our little project. I believe every slave should have a special care (one on one) and be treated to the tip of the hair. Most of the Masters and especially the Mistresses do not understand the great responsibility they have.
We can create a world, we can improve a world or we can destroy a world – so easily.

So why do they left behind?
Some Masters are tried by their slave, some do not find the same attraction, and some just do not want the trouble anymore.
My experience has revealed that there is a type of slaves that come with excess baggage and do not tell about it in advance. For example, people with mental health that try to hide their problems problem. One of my strict rules is “never lie” and if you did in advance then you get kicked out.
However, that is an unusual example and It happens very rarely.

I believe no slave should be left behind!