If you're buying a flagship Android phone, you're at the very least on 5.0, most likely on 5.1. So yeah, I still stand my point that it's not a huge issue anymore going forward.
And all I said is that I wouldn't buy an iPhone, because of the performance degradation after OS updates ("it turns to shit" as I put it nicely). It's not a fabricated statement and it's not even a debate. If it wasn't for app updates or requirements, I wouldn't have updated my iPad from iOS 6 at all. Why? Because it was working great on that OS and the newer version have zero benefits for me. I can't comment on Samsung, because I haven't owned their hardware. I didn't buy it in the past, because I thought Touchwiz was incredibly ugly. That changed since the S6 and I even considered buying it.
As for me being nitpicky, all I can say is that I'm still very happy with the performance of my Nexus 5 (a 2013 phone), which I have upgraded from 4.4.2 all the way to 6.0, without doing a factory reset. I don't know why I can't or shouldn't expect this from other devices. On the other hand, others complain about the performance on their Nexus 5 or LG G2 and G3, too. But at least I'm not claiming that it isn't there or is negligible.
That said, and to throw you a bone, I'm a very casual phone user. I posted my home screen in the Android thread. One page with eight apps on it. That's all I frequently launch and use. Zero games. So perhaps I wouldn't run into issues with an iPhone either. But after my experience with my iPad, I don't want to give Apple a chance just yet.