So sharding is essentially centralizing a part of the decentralized network?

The nodes around a particular shard will then be more vulnerable, wont they? It is impossible for me to hit up the entire ETH network, but couldnt this present a single point of entry for hackers? Also, wouldnt we see an increased possibility of any applications tied to this shard having the ability to create more damage if there are security flaws in it?

Finally, will this really enable them to scale up?