Who has suggested a 35% decrease in demand? Or, really, anything near that?
If you're suggesting that such a large drop in prices, if we're going to attribute it to changes in supply-demand realities and/or expectations (which would be a correct attribution), necessarily implies a similarly large drop in supply or increase in demand, then that suggestion is misguided. That's not how supply-demand dynamics work, that's not how markets work. Yes, it's possible in some circumstances that the percent increase or decrease in price lines up pretty well with the percent increase or decrease in supply or demand, but that's not going to be the typical case - and it's likely more happenstance than anything in those particular cases. And it's certainly not going to be the case when it comes to some markets and some products in particular - e.g., global oil.
For a number of reasons that I'd be happy to walk through with you, there is great inelasticity in global oil pricing. Small differences in the supply - demand balance, if they are (or are likely to be) sustained over a significant period of time, can mean huge differences in prices. We aren't talking about a huge (relative) spike in global oil supply or a huge (relative) drop in global oil demand. We're talking about fairly small (a percent or two perhaps) shifts in those things that have evolved over recent months and years. The reality of those shifts have hit global markets - and become unavoidable (or undeniable) - in the last 6 months of so. So, prices have adjusted as will be needed to rebalance supply and demand (again, huge price differences to effectuate smallish supply and demand adjustments). Prices have not only corrected, they've over-corrected. And, by the way, in the past such smallish (would-be) shifts have often been met with some market manipulation, so to speak (e.g. intentional cuts or increases in production), such that they didn't eventually result in large price swings. That's part of why global oil prices had been so stable over the preceding four or so years.
It can't be emphasized enough - for legitimate, natural, reasons - oil pricing happens at the margins. What's going on at the margins of supply and demand determines, for the most part, pricing. And various things happen to change the outlook at those margins, so without someone or someones actively manipulating the margins of the market, oil prices can be quite volatile. The story here (not the long-term story, but the immediate story) is that no one has stepped in (read: OPEC, Saudia Arabia in particular, has not stepped in) to offset natural developments in oil markets in order to stabilize prices.