Just like Touch ID was opened up, so will NFC.
Wrong. TouchID is a biometrical authentication system more secure than using the touch screen to sign on it for example but Apple Pay is a contactless payment system where the authentication happens via TouchID (it could happen via many other methods, such as signature, password, two-step verification, call, etc...) and the communication and security relies on NFC which was designed for the short distance communication, and now popular for contactless payments.
Since Apple wouldn't make any cent if it would allow the NFC interface to be used by anyone (let's recall that Apple hasn't invented NFC...) then that's why it won't be opened to anyone.
Apple just created a way for further revenue to its pocket thru its flagship product using current technologies. It is an extra revenue, not a exchange or shift of revenue, so Apple doesn't need to open anything as it will just get more profit.
Makes sense from both a security perspective and also preventing other NFC consortiums from piggybacking on Apple's success; i.e., "encouraging," merchants to adopt Apple's system if they want to cash in on the lucrative iPhone user base.
The iPhone has been a walled garden from the start so I don't know why anyone is that surprised NFC use is limited in the iPhone. Personally I'm glad it is because it puts security responsibilities on Apple front and center; there is no wiggle room if fraud happens because Apple Pay is Apple's baby 100%.
Merchant doesn't get anything more or less out of having Apple Pay (let's not forget it is just a 'safe' wallet for physical payments, a secure enclave to store your cards and pay with them). I don't see how a merchant can cash in the iPhone user base until Apple Pay is spread as MasterCard or VISA because you will always have to carry your cards (or even cash) with you to pay for that frequent moment when Apple Pay won't be available at that shop...
The security of the system is on NFC technology, not on Apple. Apple just put together current technologies, and I'm certain Apple will have an insurance for fraud as it has been proven that TouchID can be hacked (the NFC communication relies also on its own technology, tokenization, which is exploited by Apple to provide 'security').