Apple is not a company dealing in magic. Li-Ion batteries are the same across the spectrum, as are average energy consumptions. As far as I have seen, this type of battery is best served by storage at about 20°C and 70-80% charge. It doesn't suffer from the feared "memory effect" where constantly keeping it loaded causes damage, but in turn is very prone to deep discharge damage, where letting it run below 10% or less can have long term adverse effects. Also NEVER store these things in a fridge, nor a very warm place!
The worst offenders to battery life for me so far have been:
1) The screen itself. Even an OLED at 30% brightness was easily able to "out-suck" all other components and software.
2) WLAN, a very close second to the screen. Even if unused it still continues to scan the "ether" frequently, drawing quite a good chunk of power.
These two easily make up 50% of my phone's drain, and it was not much different for the other ones I owned so far.
Also normal is that the batteries will be at their best when fresh. I have my current Samsung Galaxy S5 for about 3 years now, and I started out having to charge it only every week or so (WLAN off and no screen usage above a few minutes a day cummulatively). By now I need to charge every 1-2 days, if using the navigating feature for longer drives (and thus constant screen uptime for hours on end) even every 4-5 hours. This would all be gone and remedied if I treated myself to a new battery - something Apple doesn't even allow you to install.
My verdict should be rather obvious.
Edit: But I strongly agree with your sentiment to holding onto your current device until either it gives up on you, or there is a feature you really really don't want to live without anymore. It is THE most sane thing to do with tech, especially with such that evolves so rapidly, especially regarding its price, no matter the brand.