At that price Dash will be generating its masternode holders $taxable profits of $50 million per week, $400 million per month, $4.8 billion per year.

Who is going to sustain that ? Who's on the other side of the accounting equation ? What happens to those profits ?

They get realised and that's what causes the crash (if the price ever gets that far). In fact near on a couple of billion per year will have to be dumped just to fund the tax on the weekly rewards.

This does not happen in fully mined coins because the growth manifests as unrealised gains instead so they are far more sustainable. Even miners costs scale with coin price. De-fi deals with it by having on-chain sinks that grow with use, so market buyers don't need to be found to sustain everything.