Based on above.
ROI 0% requires ether doubling in value in Btc terms in first year. If Btc rises 10 times in fiat, ether needs 20 times increase in fiat terms for ROI to 0%.
Every year after that, ether depreciates per table above. This implies adoption rate must exceed Btc for it to ROI positively as Btc is strongly deflationary.
What is the point of measuring Ether's ROI in terms of BTC? Why not just measure in terms of fiat, or gold? Why choose a volatile BTC?
Because a lot of people plan to invest in BTC. And there is an opportunity cost.
If Ether underperformed BTC, it's make no economic sense to convert BTC into Ether.