They wouldn't openly do it but they still could and not make it public. Besides even if they regularly and publicly exceeded 51% there's nothing in the software to self-destruct bitcoin - most people would turn a blind eye because they would have too much invested already to see it all disappear. Meanwhile ASICMiner would continue minting most of the new coins and become a monopoly that nobody could challenge.

The only way to prevent this happening is to modify the software so that miners could use either sha256 or scrypt to mine blocks - this will mean that GPU's can carry on contributing and it would be impossible for ASICMiner (or any other ASIC monopoly) to become a majority simply because even they could solve all of the sha256 blocks, the sha256 difficulty would get too high and then more blocks would be found with scrypt.