Can someone explain me why the hashrate going up is good for the price?

I mean, I know economics. I know that the supply is a function of price and cost. So if cost goes up for miners, then the supply will shrink. But why the hashrate should be consider as proxy of cost?

It's is quite possible that the cost for most of the miners is actually going down while the hashrate and difficulty are increasing. If there is a consolidation of the mining market, big mining farms are making economy of scales, so their margins increase while the hashrate increase too, so they could very well selling a greater proportion of their mined coins to enjoy their high margin.

All of that to say that I am not convince that the hashrate is a good proxy for evaluating the cost of mining BTC, therefore not necessary a good indicator for the future price of BTC.