It can not increase forever by one simple reason: the mining. Over time, energy cost for mining = revenue of mining + some small %

If BTC pris is $1 milj in 2020, it means mining revenue is like $675.000.000.000. Total world energy use is about $2000.000.000.000. So it means bitcoin mining would need expansiion of powerplants by 33% in 2 years. Thats simply not possible in such short time. And even if it was, would never be allowed by governments around the world... If bitcoin price gets too high, governments around the world will start to try to ban it simply because of the energy usage.