Why do emission-based DeFi yields always seem to collapse eventually? Is there a model that actually sustains?

Every liquidity mining project I have seen eventually collapses. The APY looks great for a few months and then the token falls and the real yield evaporates. Is this just inherent to how emission-based yield works or is there a structural fix?

I have been reading about fee-based real yield protocols like Seasons on Solana that claim to pay yield from actual transaction fees in wBTC and gold rather than in their own token. Does this actually solve the sustainability problem or is it just a different version of the same underlying issue?