A valid block that is orphaned is just as expensive to create as a block on top of the longest chain.
It makes no economic sense to create a block that has almost no chance of becoming the longest chain. Today, that costs maybe $500 per block for electricity and amortized capital to solve a block. And to DDoS you would need many many blocks, so you'ld likely be expending millions of dollars before any developers would even need to start worrying about mitigation, if there were truly a problem.
[Edit: Also see ThePiachu's comment about the checkpoint protecting against new forks emerging for blocks back when there was a low difficulty.]