I don't think someone can actually leave trading just when there are so much good projects to invest with.
What good projects are you referring to? ICO's? Altcoins with fancy features? Investing in the bankroll of new casinos? If any of these, which is most likely the case, then none of them is good enough to put your hard worked cash in. If you do however proceed to put money in them, then it's more like a gamble rather than a well thought out investment plan.
If you got into crypto trading, that means you have been into bitcoin for more than months already. I don't think you would want to back away from bitcoin for good, you will always come around.
If people quit trading due to constantly losing money, or just because they are tired of it as it is quite time consuming, then it's a decision to quit trading, and not to leave the entire Bitcoin scene. They can either keep holding their coins as a long term investment plan, or just offer their skills to the public as a service, or simply use Bitcoin as a currency tool. That's the benefit of Bitcoin, it's multifunctional tool.