If your threat model included nation-state level attackers, your only chance of success is to have the highest possible hash rate protecting it. Splitting the processing power over a bunch of diverse blockchains increases the probability that each one will fall in succession because none of them have enough hashing power to survive the attack.
Note that Namecoin is merge-mined, so splitting hashing power isn't directly a valid criticism. Rather the criticism is that Namecoin miners don't benefit from mining namecoin as directly as Bitcoin miners, screwing up the incentives, and because merge-mining is pretty much free for the miner so there isn't the incentive to mine properly. Both problems lead to Namecoin being less secure than it would be had it been designed to work on top of the Bitcoin blockchain.