A little known fact: the Namecoin blockchain's cost-adjusted hashrate [1] is the third highest in the world, after Bitcoin and Ethereum, making it unusually secure given its relative obscurity (e.g. its market capitalisation is only $10 million).
[1] hashrates can't be compared directly due to different hashing algorithms having different costs for producing a hash.
Namecoin has a number of innovations. It's the first 'alt-coin' fork from Bitcoin, and it pioneered the technique of "merge mining" where a miner could do Proof of Work on both the Bitcoin chain and the NameCoin chain simultaneously. A lot of mining pools implemented merged mining. Even though the alt-coin space has become much more crowded and noisy, NameCoin retains that early hashing advantage. It's a very secure chain.
IIRC, that's one mining pool, not one miner. The power of mining pools is relatively limited. If the workers see that the pool is attacking Namecoin and devaluing their NMC (not to mention ruining a cool project), they're liable to switch to a different pool.
[1] hashrates can't be compared directly due to different hashing algorithms having different costs for producing a hash.