> Even if you saturate a market, new people are born every day, at a greater rate than they die, so one would assume you grow or fade, unless customers just like… stop buying things in a category for some reason

My own observation is that old people tend to spend less shit on silly things, compulsive purchases and focus on the essential. With an ageing global population I don't think one can expect infinite growth unless the category of services is focused on an older population.

Also society change and some stuff that is needed now won't be needed tomorrow. I don't think there is anything wrong with a shrinking market as long as it is gradual and slow enough that it lets people working for affected companies involved to retire and/or move.