Bitcoins are never destroyed, you can only loose control of them, i.e. you can't spend them anymore! Simple fact, as long as there is a single copy of the whole blockchain anywhere on this planet or universe, all ever mined bitcoins are happily there and waiting to be spend. Guess, some of them will wait forever unfortunately...

If your last copy of your wallet.dat gets destroyed/deleted and you loose the private key(s) for your various bitcoin addresses (no brain wallet, no paper wallet, nothing left, zip, nada), then you can't spend them anymore. The bitcoins are still there, they are not destroyed, nor deleted.

Though there is still a ridiculously small, rather theoretical, chance to regain control and ability to spend by finding the appropriate private key(s) for your bitcoins. The probability is not 0, but very very [repeat 'very' as much as you like] close to zero. You can assume safely, it's currently not possible and I don't see a way this will change in the near future.