Well I've had an AIO running in a system for over 2 years and temps are as good as they were the day I installed it. I also don't see many people on here (or anywhere really) complaining about an AIO after long term use. Not saying it doesn't happen, but I just don't encounter that very often, when what you say would suggest we should have an epidemic of furious AIO users reading the riot act all over the web.
Most people who buy AIOs have no interest in maintaining a custom loop set-up, never-mind installing one in the first place. Whatever anyone says, it's significantly more complex (and expensive) going that route vs the simplicity of an AIO. I also can't avoid the obvious "well you would say that" being a Mayhems rep lol.