Nice security research. Can this attack be made profitable, or is it just DoS?
This "attack" is a nuisance just like the HighS malleability.
Is fixing of this issue requiring a full 'hard forking'
No. As trout already wrote the miners can just take a higher fee for transactions with a large number of SIGOPS.
1) You are missing that miners are interested in fees. They have a right to include/exclude any transaction.
They might be interested in the BTC value, too. So it's interesting for them to include all transactions to preserve the value of their BTC.