Why would it be? For now only just for fun hacks are possible. Who may be interested to spend money on this attack now? BitCoin is not adopted in any country. There is no wealthy bitcoin hater at this point.
The bitcoin network is worth 10 billion dollars now. There are lots of ways to profit from "hacking" the network, should it be possible. Lots of people hack for free, just to show the vulnerability in what they are hacking.
That is exactly what I say. Lots of people hack for free. What if you need some amount of money to organize attack? Even now you can raise transaction fee to unacceptable value for a day if you have 10,000 BTC. Then if the block default size will be raised you will need another billion USD to spam blockchain to the size when the current network clients won't be able to operate. This will literally switch bitcoin network off.