That's a good point. I guess with a background in maths it's "obvious" that a system with constant in-flow and an out-flow proportional to the volume would approach equilibrium, but that perhaps isn't as obvious to others.

I guess it's perhaps also worth pointing out then that the same equilibrium is reach from either side. If you start with a much higher volume of subscribers you'll reach the same equilibrium through attrition down to that number also.