This is a tenet that will lead inexperienced developers astray. This "rule" is just too ambiguous. Extensibility is a fascination for object-oriented programmers but in my experience doesn't have a lot of successful examples. Typically I have seen this manifest itself in either a Command+Composite style where every meaningful class is a composite of other SRP classes, or in a proliferation of interfaces that seldom have method definitions and are instead used to enforce coding standards or dependencies.
KISS is incompatible with this rule and you should kill this rule with fire because simple is not extensible. Perhaps when the goal is extensibility then should you consider other developers, but if you are developing a "beige" application then you should not consider extensibility. Instead, just assume that release management will handle changes, i.e. another developer will update rather than extend your class and that will be released in version 1.1.
Of course, to do this also means admitting that version 1.0 of your class was pretty much garbage and that it needed to be "extended". Tough pill to swallow for some.