So what is the point of buying aave or anything in bitcoin.
Why not just buy in euro.
I think you're confusing something here. You don't multiply your ROI by price rise on AAVE * price rise on BTC. BTC is just another pair of your coin, similar to how you use fiat to denominate the price of a phone. So either you calculate your AAVE price rise on USD or BTC, not multiplying them.
If you still don't get this, let's say you have a phone X, you bought it for $10 (equivalent to 8 euro when you bought it). Now the price in dollars rises to $30, but dollar to euro fell, so your phone is worth something like 24 euro.