Eventually the horse will die of old age. And if it hasn't procreated by then, you're left without horses.
Not all businesses will have a way of replacing a user base that is dropping off that isn't lower ROI than putting the money elsewhere.
If that's the case, then you extract the revenue that's left, but you don't reinvest much of that revenue - you might keep the horse while it's still doing useful work, but you're not building a new stable or hiring new stable boy in expectation of growth.
Not all businesses will have a way of replacing a user base that is dropping off that isn't lower ROI than putting the money elsewhere.
If that's the case, then you extract the revenue that's left, but you don't reinvest much of that revenue - you might keep the horse while it's still doing useful work, but you're not building a new stable or hiring new stable boy in expectation of growth.