"Money's value is decided by supply and demand", this statement comes from legacy financial book, and they forgot to mention the most important prerequisite of this statement: You must be monopole in making such money and force it upon its users. For commodity money like gold/silver, its value typically tied to production cost because of free flow of capitals

I can create a PoS coin and hold majority of the coins or even all of them and hope others to use it, thus give it demand and increase its value. However in open source world the competition is almost instant, if it is enough profitable, some others will also do this by using similar code, and possibly with a even higher marketing effort, eventually the demand will be spread in many different coins thus each coin's value becomes diluted

The problem in the above PoS clone scenario is inflation: If there are so many different clones out there, then the aggregate inflation will destroy each coins value and people will just leave. Many people come to cryptocurrency world seeking for protection from inflation, they will evaluate the situation and eventually only select one currency with limited supply to achieve that goal. From this point of view, any clone (including PoW coin clone) will only get minimal capital inflow