Yes, in general as app sizes increase, so do memory needs. It's not a direct correlation, but it's a correlation nonetheless.
Furthermore, as you've indicated already, memory size is extremely important for OS updates. Remember that iDevices went with 1 GB way back in 2012. It's now 2015. iPhones are long overdue for a memory upgrade.
Apple actually downgraded the RAM when they launched the iPhone 5S.
Experts have estimated ARMv8 uses about 25% more memory than 32-bit ARM when code is recompiled. So the 1GB RAM is more like 800 MB, not counting the growth of the OS itself. (I'm assuming the new larger screen sizes would also need more RAM than otherwise, but don't know how to quantify).
As for a technical reason, the old one that was given was that "excessive" RAM results in worse battery life because the memory cells draw power to refresh. That argument isn't valid if more than 1GB is not excessive, and nobody today thinks it is.
In some respects, excessive spec whoring is dumb. For example, I question whether there's any sense at all in having 500 ppi smartphone displays but Android phone manufacturers compete in a cutthroat business where consumers seemingly buy based on a bunch of specs.