All this battery issue has become a McGuffin for a serious (albeit unavoidable) issue.
Software updates make the devices slower, not because of some evil plot. It's much simpler: Software gets more complex. With new, more powerful devices, manufacturers release software with more features that need more memory (more on this later). Also applications get bigger, hence costlier to run. To add insult to injury, websites have became insane loads of javascript crap that can kill the performance of almost everything.
In the case of iOS 11 (and probably it has happened before) there were some serious bugs in the update process that somewhat hurt performance in certain devices. A backup - reset - restore cycle restored most of their original performance. Not all of course, due to the increased software complexity.
That said I've been using an iPhone 4S for more than 5 years and I stayed up to date with all the software updates. It was perfectly usable (except for browsing newspapers which, as I said, load MBs of crap) although, of course, within reason. I didn't use apps such as Facebook or Twitter which are known battery/CPU/memory hogs of course nor games.
In this thread (surprising in this forum!) there are people confusing memory with storage. And memory together with battery capacity is the limiting factor on mobile devices. The iPhone 4S had 512 MB of RAM memory and it was the state of the art at the time. The next model jumped to 1 GB.
I am still using a 2009 Macbook Pro and a 2010 Mac Pro. That's a lot of years. Why don't they get obsolete so quickly? Well, the MBPro has 4 GB of memory. Plenty for lots of applications. But the story with mobile devices is different. The 4S had 512 MB, the 5 jumped to 1 GB (double the memory!), the 6S, SE and 7 have 2 GB of memory (again, doubled!) and the 7S and X have 3 GB (a 50% increase over the previous models).
I imagine that the 2 GB models will take a longer time to have memory problems because the doubling memory capacity trend has probably ended. It makes much more sense to add 1 GB at a time if necessary. Also, 2 or 3 GB are a respectable memory size for a device that is not expected to be multitasking heavily with lots of heavy applications. But during the first ten years growth has been enormous. We'll see, I have just got an iPhone SE. Maybe I'll be cursing them in a year, who knows.
Now, before accusing Apple of being idiots(*) or scamming their customers, how are the rest of the devices? What's the useful life of a competitor's phone? How much RAM (not flash storage) do the models from the same time have? Are Apple idiots manufacturing devices limited to 512 MB or 1 GB of RAM while the rest sport 16 GB of RAM? I don't think so!
Actually they have behaved like idiots with this CPU throttling issue. And in this case all this bad press has been purely self inflicted. They should be more transparent with users. If only because of the time wasted by people affected by the slowdowns trying to diagnose the cause and doing all sorts of voodoo crap. It would have been much better to do what they have done now: be transparent, explain what and why you are doing it, and how to solve it. And I don't mean transparency just in the CPU throttling issue, but with OS update processes that, for some reason, can hurt performance seriously.