There is no economic incentive to do so so long as you're primarily invested in Bitcoins. For example, suppose someone really wealthy concluded that Bitcoins were potentially harmful to their wealth elsewhere; it might make economic sense for them to spend a few million dollars destroying Bitcoin.
I guess I should say
direct economic incentive. It is unlikely a single entity would gain more economic value than they get from destroying Bitcoin. For example if a single bank decided to kill Bitcoin then all banks would benefit but the bank spending the significant sum is unlikely to benefit by the amount it costs to launch the attack (plus any potential costs if caught).
This doesn't make Bitcoin "immune" but it does significantly raise the bar as an entity would need to be willing to destroy Bitcoin at significant cost and they may never personally reap the benefits of that cost.
For example a bank today likely could kill Bitcoin for $20M investment. However if BofA spent $20M to kill bitcoin do you think that will produce a gain in revenue for BofA >$20M? Unlikely. In the future when Bitcoin is 100x more popular the benefit to BofA might be greater but so is the cost.
There was ABSOLUTELY NO COST to kill CoiledCoin. If something has no cost to attack it will be attacked.