The thought occurred to me today that the total namecoin supply will likely ALWAYS (or for a long while) be much much less than that of the total bitcoin supply.
Here is my reasoning:
1. Once merged mining takes hold you will see the difficulties of NMC and BTC reach parity sooner rather than later thus slowing down the production of NMC blocks that are generated.
2. When the bitcoin network was at difficulty 23000 (i.e. the last NMC difficulty) there was a total supply of bitcoins of approx 5 million BTC. At difficulty 23000 of NMC on block 18144 we had (18,144 * 50) - 65,000 = 842,200 NMC. We are currently at something like 850,000 NMC.
3. At block 24,000 (the approximate time when merged mining is likely to start by) there will be approximately 1,130,000 NMC. A few difficulty increases with larger bitcoin mining pools utilizing merged mining and we may be up to 2,000,000 NMC (and we will be likely at 7.25 million BTC) when the difficulties of both networks are close to parity. When difficulty parity comes the rate of block generation will be close to one another so there will always be more total BTC than total NMC.
4. Namecoins are currently destroyed when used to register domain names (this may change).
Based on the points above it is hard to see the NMC supply exceeding the BTC supply anytime soon.
Any other angles I might be missing that someone might want to add?
EDIT: I forgot about the halving of coins per block that will occur. So we can say that for a while the supply of NMC will be less than BTC unless there is a drastic reduction in mining for bitcoins while a constant supply of NMC is mined over time.