As for centralization, if you call that alert system centralization then you should call the fact that certain people are coding for bitcoin and @laanwj is the only person merging PRs (or possibly others) a point of centralization too!
Sometimes decentralised systems are appropriate. Sometimes they aren't. Because sometimes they work. And sometimes they don't.
It's just too much of a risk to go through the Gavin Andresen situation again, it's just a waste of time that can be avoided.
Also, none of this reasoning is convincing. If soft fork BIPs just needed an alert sent on the client to get activated, it would have been very easy to get BIP142 over the signalling threshold (and this leaves the mystery of why every other soft fork was never activated that way).