I had a cashier ask me for the last four digits of my card today at BJ's. It took me a second to remember what I had to do, but I got him the information and all was well. But it is a pain; in addition to the security benefits, Apple Pay was supposed to be more convenient than using a credit card. Even though I've used Apple Pay at this BJ's dozens of times, I still had to sign for my purchase (even though it was only $60), choose "credit" in addition to giving the last four digits of the card. That's decidedly LESS convenient, since I had to spend a minute navigating through the phone to find the last four digits of the card. And this is from an Apple Pay launch partner over a year after launch. I figured that by now signatures would have gone by the wayside completely.
I'm pretty sure that things are as good as they're going to get, and only become worse as time goes on. Stores really don't care about how people pay, as long as they pay. All the launch partners figured that maybe they'd get more business if they accepted Apple Pay, but it hasn't really happened since not enough consumers care. There's no real benefit in accepting NFC payments for a business, and they lose out on getting customer data.
On the topic of training cashiers: it's got to be a pain in the ass. It's a higher-turnover job, so you constantly have new people coming in on the job. And you have all these different devices that behave differently - you can't expect businesses to have training sessions every time a smartphone maker changes something around.
Bottom line: there is no incentive for stores to accept Apple Pay / NFC payments; as there has been no lasting "halo effect". You probably won't have anyone else jump on board (did anyone BIG add support in 2015? I know that several large companies said that they were planning it, but haven't yet). It will remain a niche thing in the USA. Hardcore nerds like myself will continue to use it where we can, but it will never reach the mainstream, IMHO.