Except you profit from mined blocks. In theory you should be able do do a 51% attack for a profit even without double spending as you capture high value transactions over a longer timespan than normal. Aka in 10 ten blocks you get the same number of high value transactions as would normally occur over 20 blocks. Where you would end up with the same 10 blocks without a 51% attack but someone else would also mine 10 blocks and capture some of those high value transaction fees.