Sell advertising. The registrars will bid each other up to get first crack at the registration business. Other providers (hosting, developers, designers) will also be interested in your traffic.

GoDaddy et al might replicate your functionality. But they won't replicate a look at the competition for the business.

Better yet, be a brokerage portal. Get registrars to bid directly for the work. You also provided curated search on "best registrar" questions -- fastest, reliablest, fewest / most stolen domain problems.

If you can work out some means of obtaining good names held by squatters, that's better still -- and a real value add that would be harder to replicate. (I actually have some idea along this line.)

Why try to run a registration business when you're already good at generating traffic?

Possibly, but then it feels like you are betting your brand awareness against Godaddy's. That's a tough battle to fight. And there's nothing stopping them from implementing something similar.

I will say from first-hand experience that, technically, you are correct. However, much cash is generated through the friction of their purchasing path.

Yeah, totally true. That still doesn't make it easy for an unknown to come in an topple and established brand. That said, Google did it.. :)