It's more that a high P/E means that the company itself needs to make a lot more money for the long-term shareholders to not be holding an investment that doesn

It's more that a high P/E means that the company itself needs to make a lot more money for the long-term shareholders to not be holding an investment that doesn't pay off in the end. You can still make plenty of money speculating on it as it's going up (so long as you sell), or short-selling it as it's going down, but generally speaking it's a sign that the business is being valued the assumption that it will grow by a lot, which is riskier (for example, Tesla's valuation more or less requires that they eventually monopolise the car market or enter and dominate some similarly large market. You can make an assessment yourself as to whether that's likely to happen if you want to make a prediction for how Tesla's value is likely to change over the long term).