Now if you had variable loads or a battery you could schedule them to buy and sell power based on the market rate. Since very few people have the ability to do this, you can constantly beat the market on an almost daily basis. Tesla in fact already does this. Not only do they profit from this buying and selling, the grid becomes stabilized because the high and low demand periods become smoothed.
That's incorrect usage of terminology because energy is not an investment. Nobody buys a kWh of energy expecting it to net a return year over year. It's used as a consumable like food or water. If you buy low and sell high you are engaging in trading, not investment.
Trading is about providing liquidity and the maximum liquidity a market needs depends on its size, not the number of traders. Those traders basically have to share a market and if there are too many traders then the profit per trader will go down. This is the biggest reason why trading (not investment) is zero sum. At some point enough is enough but this isn't necessarily as bad as many people portray.
A lot of industries are like this. Once everyone got a smartphone or PC the only way to grow faster is to displace a competitor. Trading in the stock market is usually a poor choice because there are plenty of professional businesses that are already providing enough liquidity.
Trading in the energy market can be a good choice precisely because the market isn't crowded as you said.
If you dig around enough, you'll find plenty of stories of variable electricity costs. From what I understand, it's more common for business customers instead of residential. (Apparently chillers that freeze water overnight with cheap electricity to run daytime air conditioning is a thing.)
But anyway, I have a fancy powerwall and solar, but I only use it for backup. Why? There's no incentive to use the battery. It's 95% efficient, so if I use the battery at night, I get less net metering credits than if I just use it as backup.