Don't expect close to same difficulty mid-term.
Namecoin attracts less users because there isn't that much demand for it's intended usage in comparison to bitcoin. And it is much less user friendly, doesn't come with a gui client and adaptation/ easy access to the infrastructure is lagging behind. But there could be a speculative bubble which wouldn't last very long but it would bring more attention and later, after the collapse of it better adaptation.
What does any of that have to do with difficulty.
One pool is already setting up auto-conversion. The pool merge mines BTC & NMC. For miners that don't want NMC it will sell them for BTC and increase the miner's BTC balance.
Given there is minimal logistical overhead to merged mining (once setup) I would expect most pools to merge mine eventually. Difficulty on NMC is going up up and up.