A catalogue is not one income stream that fades. It is three or four, each fading at its own speed.
Once the income is normalised to a clean , a buyer has to answer the next question: how fast does that run-rate fall away? The forecast (and therefore the price) lives entirely in the .
The trap is to treat the catalogue as a single blob of income declining at one rate. It does not. The same £200k-a-year catalogue is really a streaming stream, a performance stream, a sync stream, and a tail of others, and each one behaves completely differently as the songs age.
Streaming decays fast at first then flattens. Performance income from radio and broadcast is slow and sticky, established songs keep getting played for decades. Sync does not decay on a curve at all; it lurches up and down with individual placements. Blend them into one rate and you misprice the catalogue, usually by getting the mix between fast-fading and durable income wrong.
The discipline of this episode: model decay source by source, then add the pieces back up. Never one curve for the whole thing.