The symmetry is irrelevant when the attacker has stolen someone else's resources. Granny may notice her computer is even slower than usual, what then?
If we stipulate a future where attackers cannot steal other people's resources, then work backwards, I can't see any internet with any degree of freedom.
It would be necessary for every system to be 100% watertight. Mathematically-provably so. Spectre/Meltdown demonstrates that this isn't just formal proof about software. Eradicating the pathways to ddos is an intractable problem. You'd need to seriously consider preventing all network access except through certified and regulated kiosks to which users do not have direct physical access. Like, hand a librarian a piece of paper with a URL on it and get a print out.
I'm not expert, you shouldn't have confidence that I'm correct.