If the money supply is fixed, then people will have a harder time acquiring namecoin after all the namecoin is generated. People will have to buy it, and since it's a scarce resource, it may become extremely expensive. Especially if namecoin exploded in popularity.

I suppose if it becomes expensive then the namecoin admins could lower the cost of database inserts/updates. But it seems like that would prompt the price per namecoin to rise accordingly, because the value of namecoin is a single database insert or update.