And the features on that phone is fragmented and crap and performance suffers. As many have stated in the youtube comments of the video below, planned obsolescence is real with everyone.
https://www.youtube.com/watch?v=jvj_2Yn6aRY
And I think the 4S shipped with IOS5, so it's only seen 4 updates. Also, major updates don't mean a think if you are releasing major bits with incremental updates. We've only seen 8 and 8.1 because they were keeping the illusion of the one OS..lol.
Again, I don't think that Microsoft should have tried to update all of those phones, especially the budget phones with 512MB of RAM and low end CPU. They maybe could have optimized a little more, but my 928 ran like crap for a long time with Windows 10. That was not an enjoyable experience.