You obviously don't know much on how electronics and software works right.
What a compelling argument.

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Of course it should be just like the day it was bought even if the software has been updated.
Not saying it should, but it certainly isn't proportional. There's no reason Apple can't make devices run better for longer, or at the very least let people revert back if their device gets completely ruined by an "update" that was nearly forced upon them and they had no idea how badly it would affect their device.
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You mentioned it yourself - they all work perfectly on the software of their time - is your iPhone 6 running the version of iOS it was released with?
I wish! For some reason, Apple goes to great lengths to make sure I can't get the device back to a version that ran beautifully on it.

I did manage to go straight back from 11 to 10 on it, at least.
I’m not too sure how technically knowledgeable you are in terms of these things but when you say - what has changed?! - I’d recommend you read the developer documentation on the Apple’s developer site.
I agree that basic functionality such as slow typing and scrolling makes you feel like that’s the only thing the device is doing but over 80% of OS changes are behind the scene mostly. Consumers don’t see these things. Even a basic change of indexing algorithm might have an impact on these things.
If all the under-the-hood changes result in a device that's 500% slower in every way, are they good changes for that device?
I mean, what's changed in the camera app to make it take 5x longer to open than my 4S does it, which is like 1/8 as powerful? Or about 5x longer to open the Music app and interact with it?
If the phone is 8 times more powerful and takes 5 times longer, that's saying the software demand is 40 times greater... and I don't consider that a reasonable increase in software demand for such similar tasks. That's really the point I'm trying to drive home here.
From a pure business perspective it just doesn’t make sense for Apple to spend a lot of time and energy behind optimising the OS for older devices simply because most consumers upgrade their devices either yearly or in a couple of years max.
Having said that Apple is the only company in the consumer electronics market sector, specifically with smartphones and computers that support devices that are 5 years old at least. That’s a long time in tech industry. Very long time.!
Maybe people wouldn't upgrade if the software still worked properly. The hardware is barely changing these days, compared with the early iPhones. My iPhone 6 still has the same overall form factor as the 8, even.
If the 6 still ran beautifully, why would people go spend $800 to replace it just to do exactly the same thing almost exactly as well as the device it replaced?
It's one thing to support devices a long time, it's another to nearly (and sometimes actually) force devices onto software that renders them garbage. I'd rather they just not bother and let the device age out of date than to crush it.
Keep in mind this doesn't always have to be the case. My 2010 17" MBP is still "supported" by macOS, but Sierra and High Sierra run terribly on it. El Capitan is adequate but not great, but security updates are ended. However, Windows 7 and 10 both run absolutely great on it and are supported for years to come.
The idea that software must slow down even quite new devices really is based on a time when computing advancements were far more steep. Now that the devices are matured and changes are more and more fine, it's possible to produce software that's even more refined and efficient than ever.
Of course, 1GB RAM limitations on my iPhone 6 is very obviously a problem -- I knew that shortly after I got it, no matter how many Apple fanatics insisted that Apple's magical memory management couldn't possibly need more than 1 GB.
