1. less of an issue as time goes on. regardless, not much can be done with a 51% attack anyway.
2. not really an issue. every argument ive seen about centralization misses key points on how they are wrong.
3. Pruning.
1. ASICs & Mining facilities. 51% lets you do anything from ruining the currency to implementing modifications to the protocol.
2. Centralization makes it a lot easier to make a 51% attack, but besides that, centralization could result in governments taking over bitcoin and destroying every single one of its benefits over fiat.
3. Reduces the security of bitcoin.