How realistic(practical) this attack is ?
Very doable if you can afford to spend 10/15 millions of $
Any rich guy could happily do it without problems...
But due to Sathoshi's genius clever design, would-be-attackers are instead incentivised to use their resources as legit miners instead, thus increasing the strength of the main block chain.
Yes, private attackers could be persuaded by the reward for honest mining. But banks and governments who are willing to invest to just shut down bitcoin are still a danger we should be aware of. I don't see any chance for them to get sabotage done, and it will get harder as bitcoin gets more users and miners. But we should always have the possibility of attackers in mind, who don't care about money but about hurting bitcoin.