More hashrate means it's more difficult for someone to rewrite the history of bitcoin transactions that are agreed on globally. It defends against attacks on the system's consistency. This is the central idea behind bitcoin.
Discussion
But it also means Bitcoin is pricier .. and thus the incentive to hack is far greater .. is it not ?
There's no guarantee that the price will move in the same direction as the hash rate.
But you're of course right that the incentive to create an attack like a double spend (you pay for something with btc, then fork the blockchain using hashpower, you change history so that your spend is 'cancelled', then you have the thing and still have the btc) increases as the purchasing power of btc increases.
I guess incentive to hack depends more on amount of txn - txn value must be higher than the cost of hacking !
Possibly, though of course it could be many txs not one.
But the exact nature and motivation of an attack is hard to predict. Another motivation might be to try to kill bitcoin by making it unreliable. That attack is quite hard to reason about.