Whether it's a completely new device built from scratch, the experience is exactly the same as an iPhone 5.
You reckon?
Mmmm, not convinced. I upgraded from a 5c, chalk and cheese for me...
1.) The screen is significantly bigger and more vibrant than that on my old 5c. Working on it is an absolute revelation for a long time iPhone user that had to be content with 4" or less. For me, the debate could end there but for the sake of substance I'll continue.
2.) It has a fingerprint sensor for unlocking which initially I believed to be a gimmick and now I use exclusively, no such experience/option on my old 5c
3.) I thought the 8mp shooter was quite underwhelming considering the 20mp plus options available from the competitors, but no, it shoots beautiful pics with a marked improvement over one's captured on my old 5c. The few reviews and comparisons I've read, the 6/6+ more than holds its own against the S5, Note 4, LG3, Xperia's etc etc etc
4.) Reachability is another new feature which has come in very handy, excuse the pun.
5.) I reiterate, your perception that the experience on a 5 and 6 is exactly the same is a huge credit to iOS. Not everyone would agree with you, Dolby for example...
6.) Generally the 6 is far more responsive than the 5/5c.
7.) Although for me battery life hasn't improved much between the 5c and 6, a lot of users have noticed a significant improvement in this department. That weighs heavily on the over all user experience.
So ja, having used both I'm quite surprised you've experienced no difference, either physically or from day to day use.