GRs Cousin, you're right that this lower-and-lower-tier game could go on forever in theory (or, at the very least, for another six rounds, assuming 50 pokemon per tier), but it won't, because tiers are based on simulator usage, and right now, the upper staff has decided that nothing below NU will get a ladder (the ability to find random matchups) on the simulators.
The reason for this is that every new usage-based tier saps off usage from other tiers. The best example of this is the birth of NU, which cut the RU userbase down by more than half.
So NU will be the last tier implemented on the simulators, which means that we can only come up with a tier list for one tier beyond that.
So no, there won't be a DU (a name I like better than my idea--FU, which is what those Pokemon say to GameFreak) or any tiers beyond that.