I believe that Walmart is going to have to reverse their decision to not accept Apple Pay. If grocery chains like Public, Kroger, Ingles, etc. get on board Walmart will be forced to join in.
If I had to guess, I'd say that Walmart probably demanded a better deal than everyone else. Walmart uses its size as a huge negotiating point when dealing with suppliers. In Apple's position, I'd mount a campaign showing people using Apple Pay at merchants and choosing to shop only at places that take it.
Personally, after I see that it works well, I'm planning to no longer carry a wallet. Just a simple ID folder will work.