I'm thinking PoW really needs decentralized mining to truly be safe.

You know, that's like saying: "We need to stop all the wars in the world and cure all diseases". Easier said than done.

Sorry but I think I disagree.
Quite on the contrary, I think that the problem with mining centralization can be effectively considered a flaw of the protocol and it has to be resolved somehow.

The greed is something inherent the human being, the man, the miner, the pool operator. Bitcoin must be superior to that.
We cannot just hope that these actors will change their minds and won't be greedy anymore, because it will not happen.
Sometime there is someone with good intentions, like the recent GHash.IO statement, but at all times it could appear a player greedier than the previous one. Also, we should consider that even if greed was totally absent, the problem would still be here because anyone with enought resources could be able to destroy bitcoin for whatever reason.

No, I really think that it should be the protocol itself able to auto-defends and to defeat any threat.

Because humans are lazy.

This is always a possibility. You could say the same thing if 5 pools with 11% each were to work together to attack the network. This is a non-issue.

It's a huge issue but the community has accepted the fact that we rely on trusting the miners. I've posted about this issue before, about how Bitcoin is not a 'trustless' currency at all. And I'm not saying that it won't function perfectly fine for the foreseeable future, but it's more convenient for people to ignore the issue in general. There is no easy answer really unless the devs could implement a forced p2pool mining system into the the protocol. Which I assume is not possible since it's such an easy answer, is it not?

The protocol is open source though, which makes any attempt to completely thwart the effects of human greed simply a delaying deterrent at best. Also, any theoretical principle will always play out differently when applied in the real world, which means that even a perfect protocol will eventually find itself abused and exploited. So the mining process is adequately functional as it is now, in my opinion.

If you want to keep the blockchain as decentralized as possible independent of people's behaviours, then you would need to rewrite a large part of, if not the whole, protocol and make it closed source. That's obviously out of the question. Ergo, it's the miners themselves who have to change.