Are traders 100% responsible for a stock's price changes?

Traders = every market participant. Not some shadow figure that excludes you just because you passively drop cash into a 401k Vanguard fund every paycheck.

So yes, if everyone stopped trading then the price won't move. Trades are 100% responsible for the prices you see on charts and tickers.

A stock won't be worth "$100" if nobody ever traded $100 for it. It only has that price now or in the past because somebody placed an order for it at $100 and somebody else filled that order at $100

When people talk about "the price" of a stock, they usually mean one of the following:

  • Last price: The price at which a trade most recently took place. If someone sold (and someone else bought) shares of XYZ for $20 each, then until another trade occurs, the last price of the stock will be quoted at $20.

  • Bid price: The highest price at which someone is currently offering to buy the stock.

  • Ask price: The lowest price at which someone is currently offering to sell the stock.

As you can see, all of these are completely determined by the people buying and selling the stock.

Value is the key word here.

Traders should ideally trade on the perceived future value of a company. Changes in the perceived future value is what leads them to buy and sell shares.

That said, if a company were to have some catastrophe happen (say it and all of its employees and property disappeared) and somehow every shareholder agreed to not sell, the companies market capitalization would remain unmoved even though the value of the company is gone.

So theoretically yes, but it is unlikely.