actually hashrate does indicate adoption but not the way OP wants. it is showing the past not the future. basically it works like this:
adoption > price rise > mining becomes more profitable > more miners come > hashrate rises.
Agree, but there are different types of adoption with a different influence in price.
- Adoption of Bitcoin as a means to save money ("hodl") tends to rise the price, as it decreases the amount of BTC available on the markets.
- Adoption "as a currency" tends to stabilize the price on a high level, as it adds supply and demand (thicker order books), so it leads basically to more liquidity. We're still not seeing that in a massive way.
- Adoption as a means of speculation (people trying to buy low and sell high) is actually much more popular than HODLing. It tends to create price volatility in a cyclic manner, with sharp FOMO-fueled increases followed by panic sells. That's the reason why we see such drastic price changes.
What does that mean for hashrate? First of all, again, it is to expect that hashrate increases 50% per year if the price is stable because of Moore's Law as I explained
here. HODLing would definitively increase hashrate more than that, because of the price increase, but it's actually less popular than it is thought commonly. Adoption "as a currency" would lead to a slightly over-average increase because it decreases the risk of miners and increases also transaction fees. Speculative adoption leads to increases and then stagnation or even temporary decreases, like in 2018.