I'm finding a problem with the idea that when a dating site is very successful, they lose because the participants immediately become non customers.
The same is true (kind of) with car salesman and real estate salesman, if they are successful at selling the customer a car or house and they do a terrific job, they won't see that same customer again for 8 to 30 years. However, if the customer is delighted, word of mouth spreads, and you get a flood of the customer's friends.
True, but people rarely brag that they met their partner online. Relatively little word of mouth.
As for car sales -- this is a field notorious for how salespeople con the customer, probably for the reasons you specify. People pick the type of car they want, and then endure the shenanigans at the dealers of that brand in their area.
Obviously it's anecdotal, but a large portion of my friend circle have met their partners online, and there's been a strong word of mouth component there. For instance, I met my girlfriend on OkCupid, which she was only on because a few of her friends had just found boyfriends/girlfriends on the site and she figured she'd give it a shot.
The same is true (kind of) with car salesman and real estate salesman, if they are successful at selling the customer a car or house and they do a terrific job, they won't see that same customer again for 8 to 30 years. However, if the customer is delighted, word of mouth spreads, and you get a flood of the customer's friends.