Lol I feel you because I just upgraded to a 6 Plus this past Christmas, before I was still using my 6 year old iPhone 3G, and I plan to use my 6 Plus for years as well >__<
From my experience, iDevices' longevity can be quite impressive if you are not a heavy user and don't feel the need to always have the latest device. I also own an old iPad 2, I've always updated it to the latest iOS release available, and it still runs acceptably smoothly and fast with iOS 8, it starts to lag a bit only when I open a lot of tabs on Safari. I think that originally it was running iOS 5 when it came out, so you shouldn't worry about compatibility with future iOS releases for quite a few years. Even my super old iPhone 3G, I kept updating its iOS for years and the hardware back then was really way ******** than now. Obviously, your device's performances will get worse with any iOS update, it will get a little slower and a little buggier each time, but if you're not a heavy user it might not be much of a problem really. Normally, I use my iPhone for messages, calls, listening to music, taking pictures and editing them, checking social media sites and browsing the internet, my iPad for internet and writing, and I only rarely play games or watch long videos, so if you're like me I don't think you will notice any difference in performances at least for a couple of iOS updates.
Apart from the software, I've found that iDevices' hardware generally is pretty durable indeed. I treat my devices with great care, putting them in cases and using protective films for my phone's screen, but I've dropped them lots of times and accidentally scratched them in my pockets/bags during the years, and yet my iPad after 3 years is still in pristine conditions, and my iPhone 3G's home and lock button started to have serious problems only after 4-5 years of usage - they both still work but sometimes I need to push them harder a few times before getting a proper response. Battery lifespan is alright too, my iPad 2's battery life is still the same as new because probably I haven't even reached its maximum charge cycle count yet since I charge it every 3 days or so, while old iPhone 3G still can last for a couple of days of standby with data service and wifi on.
Personally, I don't like to upgrade my devices often. I get quite attached to them even if I know it's stupid, but they become part of my daily life as almost an extension of myself, so I can't help but grow fond of them, their looks and design, and I want to use them as long as possible until they really aren't able to function properly for what I need them for.
So yeah, I don't think you'll have anything to worry about for a lot of time. You'll definitely enjoy your new iPhone for years, and I hope to do just the same