Of course increasing RC costs would make the flood attack harder, but first and foremost would also make a lot of users dependent on delegations. Increasing RC costs would be like making one of the damages of successful flood attack the reality without any attack occurring.
I'm on a position that even free users should have enough of their own RC to do basic transactions (free users are not actually free, since there is fee or massive RC cost for the creator). They absolutely must be able to make authority updates, but they should also be able to use all their allotted daily votes or write an article of reasonable length (the only way to acquire stake without buying). Current RC costs are almost too high for that. If the typical traffic on Hive increased to the point when witnesses would decide to increase max block size, we are going to need to work on issue #679 - I've actually did most of the work already, except the actual values of scaled parameters, because it requires very long test runs to observe how RC costs are changing with different params and traffic levels (and yes, the solution allows witnesses to keep high RC costs despite allowing bigger blocks, but they'd also need to keep in mind that budgets meant for partially filled 64kB blocks only allow for 10 hours of saturated traffic on 2MB blocks and then all the RC in the system is consumed, you need to be blocktrades to do a transaction at that point).