why can't we swap one desire for another and another? We know that because of impermanence, that nothing is worth clinging to. But did the Buddha say any words on why we cannot swap one impermanent source of satisfaction for another, and continue doing so?

For example, one might take drug A to induce euphoria, then when drug A wears off, replace it by drug B, and so on.
MN 75 gives the analogy of a leper and shows that by swapping one desire for another, we would simply be making our craving and attachments progressively worse, which is a slippery slope to deepening our suffering.

The reason for this is that sensual pleasures are impermanent, and either their future absence will cause suffering, or their future side effects will cause suffering. They can't be sustained forever. And when you swap one desire for another, craving gets deepened and the "sensual fever" burns deeper. This will result in suffering becoming deeper.


  "Now suppose that there was a leper covered with sores &amp; infections,
  devoured by worms, picking the scabs off the openings of his wounds
  with his nails, cauterizing his body over a pit of glowing embers. The
  more he cauterized his body over the pit of glowing embers, the more
  disgusting, foul-smelling, &amp; putrid the openings of his wounds would
  become, and yet he would feel a modicum of enjoyment &amp; satisfaction
  because of the itchiness of his wounds. In the same way, beings not
  free from passion for sensual pleasures — devoured by sensual craving,
  burning with sensual fever — indulge in sensual pleasures. The more
  they indulge in sensual pleasures, the more their sensual craving
  increases and the more they burn with sensual fever, and yet they feel
  a modicum of enjoyment &amp; satisfaction dependent on the five strings of
  sensuality.