As far as I've understood an attacker can destroy the Bitcoin-network if he solves several blocks in a row.
No, the attacker could reverse transactions within the last hour or so. That would lead to double spending, but wouldn't be a complete destruction.
Merchants with high value transactions would be recommended to wait for more than 6 confirms.
Waiting more confirms gives better security, if miners started to reverse transactions.
What I wonder is
1) Is my premise wrong? Can bitcoin be destroyed if one attacker mines several blocks in a row?
2) Is there something else that makes 51% the threshold for being able to destroy the network?
The point is that with < 50% of the hashing power, you can sometimes reverse a small number of blocks.
51% is a shorthand to mean more than 50%. With > 50%, you can reverse an unlimited number of blocks.
In theory, you could reverse all transactions since the last checkpoint.
There is less risk. An attacker with more than 50% can reverse transactions with certainty, while one with less than 50% has only a probability.
There are attacks that allow a miner with > 50% to break the difficulty system and create an unlimited number of blocks very quickly. This would allow the miner to take all the remaining minting fees.