You seem to have completely missed my point. Let me try an analogy: If reliable cars sell better, car manufacturers are incentivized to make their cars more reliable for the cost than their competitors ad infinitum. If a car is infinitely reliable, they never get repeat purchases (clearly they should start upcharging motor oil like printer ink). However, we're so far from perfect reliability that on the margin it still makes sense for any given car developer to try to compete with others to make their cars more reliable.

That doesn't take into account damage to cars or relationships from car accidents. It also doesn't account for polyamory or owning more than one car.

If okcupid's saturation level was 90 percent of the single population, that would be one thing, but there's WAY more marketing to do before that could ever happen and having a good algorithm is basically their entire (theoretical)advantage.