As far as updates go, Samsung needs to broaden their vision, as Apple releases updates for pretty old gadgets; I recollect when I owned an iPad, it received an iOS upgrade even during its fourth year, that too during a universal rollout.
I also agree what you've written about battery performance reduction after every OS upgrade. But you have to realise that the battery consumes a lot of power if it has to search for a network / mobile tower; if your smartphone is within range of the mobile towers and gets strong signals, rest assured that your smartphone battery will provide a long backup and long screen on time.
As far as the processor is concerned, I'm satisfied with Exynos and wouldn't be perturbed if my smartphone had the Snapdragon.
I'm not a big fan of Apple and will continue using Samsung Galaxy S series smartphones.
the problem is not from samsung the problem is from google its self Google does not want the device to be upgraded to latest os ask google to break the policy of two major updates only this also the android is bit much fragmented so it is hard for google to maintain the os update.
ios devices receive 4 to 5 year update because they have uniformity secondly their software optimisation is far better from android device thirdly they dont give their os to third party manufacturers.
but in android the problem with google is that google is giving up their os to third party manufacturers in order to customize the android according to their needs . so my conclusion is that ask google team that they should not provide their source code openly instead they should make their own os that could only be controlled by google only . these are the reasons why android is not successful in long term how ever you can us any outdated android os without any problem . as for the security concern I would tell you that dont root the devices . by rooting the devices you make the mobile is vulnerable to attackers so that they could attack and can take all of your data .