My thought is that RAM is a problem for updates. The iPhones 6 in the wild I see are dogs (1GB of RAM), but the iPhones 6S (like mine) still seem great (2GB of RAM).
The iPhone 8 also has 2GB.
My theory is that in terms of iOS updates the 6S has a few years left in it. It performs great on iOS11 and I am hopeful that it’ll be great on iOS12 too, especially as Apple are concentrating on performance and reliability at the moment.
Well sometimes It isnt only on ram. You have to consider 6 Things also:
1.
Year and old of device- Dis is the most important, the older the device is the lesser chance of getting newer iOS version
2. Processor and its Chip - Such as lower NM to avoid overheating. 6S still its NM high making it prone to overheat when running heavy apps unlike newer iPhone has lower NM.
3. Ram - this is only make device capable running apps to reduce reloading
4. Clocked rate of Gigahertz- does it have extracores? Is it underclocked or overclocked? Or only dual core like da iPhone 6S/6 and below? Core can be a factor whether it will support more iOS version just like the 4 and 4s before
5. Features- such as hevc encoding, water resistant, hexacores, dolby vision, bluetooth 5.0 that 6S lacks
6. Power Efficiency- such as higher battery capacity, uses less power.
If Ram is the only key for an iphone and iPad to support latest version how come iPad 3 with 1 GB of ram did not get iOS 10 even the iPad 3 has the same ram of iPhone 6? Or iPad 4? See??