When Bitcoins stop being generated it won't be a problem: prices can gradually be lower. Namecoins, however, have a more-or-less set price, which is the domain register price. If Namecoins stop being generated, but people of course still want to register domains, what happens?
The Namecoin block reward is to be halved every 4 years, similar to Bitcoins. The fee to register a domain in Namecoin is set to decrease by the factor of 2 every two months, and later by a factor of 4.
As the price of registering a domain go down way faster than the block reward, there won't be a shortage of opportunities for registering domains. On the contrary, soon the price of a domain might be so cheap as to make them almost free. As Namecoins are destroyed though, eventually in the far future one might expect to run out of Namecoins, but with the price/reward ratio decreasing so fast, the project can become obsolete by then.
This seems to create a lack of balance of the Namecoin protocol that can be addressed in the future.
Probably the same thing will happen as with Bitcoin. So far about 3% of the generated coins have been destroyed. By now the price for name operations is fixed at 0.01NMC so the percentage of destroyed coins should decrease for a while. If there is a problem the system could be modified. IMHO the destroyed coins will be completely neglectable for a long time.