It is not something unique to crypto though,,, the adage is something applied to every single investment and in fact if you read every contract that you sign with an investment (traditional like bonds or stocks) there is even the same line of advice there.
People think this risk is only in crypto but it is in EVERY investment,,, never forget this and recall it whenever you see someone claim to guarantee profits.
I agree with you. But most people forget about that because they have greediness, especially if they see they can make a big profit from crypto, which is happening to many people. If they can remember that the risk will always be there in any investment and be careful when they put their money in the investment, they will not break that and will enjoy their profit while controlling their greediness. So the adage will always work in any investment and we need to calculate the money you will be using for the investment.
But that greed is really less about being crypto and more about them making money. They would act exactly the same if they were trading stocks or gold or silver,,, same way, using maximum leverage and using maximum loans if they could.
Trust me, these guys will go into Defi and NFT in the exact same way,,, not just because it is crypto but because it is promising them easy money. If something bigger and more lucrative (in their thoughts) than crypto, they'll go there.
At least, if they can control their greediness, that thing will not happen and they will have a chance to make money. Whether they trade in crypto or stocks or gold or silver, controlling the greediness will be necessary and the important thing if you want to make a profit. Otherwise, you will only lose your money and once you lose the money, it will not be easy to recover and it could need long before you can recover the money.