Coordination of peers/forger into a voting block is not decentralized.
Not all Nxt holders will be involved in the decision-making process, and the smaller shareholders and retail partners will be alienated - even if such a measure reaches 51%.
In the case of retailers, they could potentially face loses for services rendered or products sold.
In view of that, what major investor or retailer would dare make any future commitments to Nxt if they know Nxt has a history of doing rollbacks?
Would a major retailer be willing to offer a Nxt payment option to their customers knowing a rollback is a possibility?
In crypto currencies the majority of miners/forgers decide everthing. It's 51% or so in PoW and in NXT at this time. When TF is implemented in NXT, then it will take 90% power to overcome the chain, which will make it more secure than PoW. Bitcoin did rollbacks in its history too, so nothing dramatic here.
Yes, but that doesn't mean it is decentralized. In fact, centralization of mining power is one of the biggest concerns for BTC now. Further, I don't think BTC protocol allows rollbacks.