The reasons why we are moving to a more centralized internet are what is interesting, such as - you rightly identified those - that stuff isn't always powered up and that keeping a mailserver up and running is work and so on.
But none of that changes that centralization is happening.
Multiple distinct companies != peer-to-peer internet. That's what a decentralized network infrastructure used to mean, where the 'peers' were equals.
Nowadays it means clients in one camp and servers in another, and large scale consolidation of those servers in the datawarehouses of a relatively low number of companies serving up the bulk of the data. If that trend continues it's not a bad or a good thing per-se but it would be good to stop and think about how desirable that is.
So from that point of view a lot of centralization has already happened.
Everybody running their own mailserver: could be a good thing, presuming they can be made easy to set up and easy to maintain (I don't see any technical reason why not). Ditto webhosting, why should facebook host all your content (or google, or Yahoo).
In the end, convenience won over 'peer-to-peer', there are many reasons besides convenience (firewalls, for one) but the results are here and we'll have to live with it (except for a couple of die-hard hold-outs).