During the Bitcoin bullmarket we saw price spikes carrying the Bitcoin price significantly higher than trend. However those price spikes become smaller and smaller as time goes on.
It is natural that in bull market, models can give over estimation while in bear market, models can give underestimation and sometimes there are wild periods that make good models seem to be broken, and invalid.
Your discovery is interesting but it can be explained, with higher price, bigger market cap, it's understandable that later market cycles and future cycles will bring smaller ROIs than past cycles.
This chart of Mayer Multiple reflects this fact too.
https://charts.bitbo.io/mayermultiple/
