If you truly believe that working harder, while still working as smart as you can, doesn't significantly aid your efforts, you're simply deluding yourself, and self-delusion always leads to suboptimal results.
The public record is rife with minutely detailed cases of hard work leading to out-sized success. Hard work by people who were already working very smart, as smart as they could.
Yes, other people can work really hard, too, and so it can be an unwinnable race. Sadly (perhaps), that's life. That doesn't change the fact that working harder is under your control, being smarter isn't.
Yes, some people get burned out. Yes, taking a break can help some people find insights they wouldn't have otherwise. However, some people don't get burned out. If you don't realize this, you're fooling yourself.
Bobby Fischer is the only American to be world chess champion. Read about the complete immersion in chess that was required to achieve this.
It takes 10,000 hours of directed practice (directed practice means "working smarter") to become an expert at something. No amount of working smarter makes the 10,000 hours go away.