Actually it does take time to get consensus for a transaction. So if one tries too great a transaction rate then they fail.
So for an actor to attempt to use safecoins to overload the system then they will need a ton of safecoins and accounts/wallets
This in itself is very much a limiting factor. Even using a botnet the bad actor needs tons of coins. And the larger the network the more coins they will need. We are not talking of 10 or 20 coins but millions and even at 20 cents each the bad actor to attack a reasonable sized network is needing 100(s) of thousands of $$$ to do the attack. Only when the network is very small (1000's of nodes) an attack might just be affordable using safe coins.
It is much cheaper to keep mutating MDs and pay the PUT cost. And do it when the PUT cost is very very cheap.