How Future-proof? Very.
I admit that until fairly recently, I was firmly in the 'iPhone 6/6 Plus not being future-proof' camp for the very reason (RAM) you have doubts. Then I reevaluated my priorities and decided that the iPhone 6 models will be the new baseline for all phones.
That is because the world is forcing us to no longer be 'average' users. Security breaches have made password managers, themselves accessed using a good password, a very strong recommendation. To keep from being driven crazy by continually having to enter a strong lockscreen password, then entering in another strong password to access your password manager, Touch ID or it's Android equivalent is becoming a must. The same for mobile payments. I doubt merchants can keep our credit/debit card data, and the associated information, safe. Tokenization is also recommended and Apple Pay/Google Wallet via NFC is currently the best model.
Those two features are now my highest priority and have made me decide, want it or not, my wife is not going to inherit my iPhone 5 when I upgrade, but will get an iPhone 6 for Christmas. Surprise!