Hard disagree:
I understand it's maybe a bit of an uphill battle to support a software stack that the OS/hardware vendor itself doesn't support, but:
1. telegram/whatsapp do, so it's definitely possible.
2. The point is not to add new features, but just to leave the current version be without deprecating it.
There is probably a bit of a development burden in keeping old versions up, but I don't think it's that big: they could even let the community take care of that…
3. But most importantly, "the iphone 6 had a good run", so the natural continuation to that sentence is "so let's discard all those working devices" ? wtf?
I think that from a company that defends "public interests", some more thought should be given to not wasting energy, time and money forcing people to upgrade working smartphones…