I am using an HTC Desire 826 from 2016 running Android 6, while my father is using a Galaxy Note 3 Neo running Android 4.3.
I personally don't have much issues with regards to not having security patches.. the phone itself does most of what I want, although I am having issues with other things unrelated to security patches -- it's getting a bit sluggish, the 16GB storage does get annoying, and the battery barely lasts two hours of web browsing. Also, it no longer accepts SIM cards for whatever reason. But it's not a big deal to warrant spending money (I don't get called at all and I do have a dumb phone that I go out with for emergencies. Besides, I no longer get annoying messages and calls from my carrier)
Similarly, my father's is doing alright, and the issues he is facing are unrelated to security patches -- many apps don't support Android versions older than 6.. some of them are essential things like Google stuff. YouTube and Gmail for instance outright don't work, and Chrome occasionally brings up messages along the lines of "Please update your Chrome version". But nothing is caused by not having security patches.
What am I missing? This is a genuine question.