I don't think there is anyone that will say that trading is completely bad because it isn't and there are people making a decent living from trading. What seems to make trading bad is the attitude of people towards trading. If you have tried trading before, you will agree with me that there is this hype and tendency to think that making money could be easy and fast from trading. This is usually the source of problem for people and the reason you see many people fail in trading. When people are so hyped and focused on the money, they tend to ignore the need to control the risk, therefore they go all in and often harm their finances so badly.
The only trading strategy which have potential to work in an unpredictable market is long term trading, which involved the trader holding for a long time period, up to few years... That is why patience is needed. On the other hand, for daily traders there aren't any assurances they are going to make guaranteed profit from this activity in a daily basis, therefore to rely a living from trading is too risky due to market's conditions.
And doesn't matter how professional and skilled the individual is on trading matters, he still can't forecast the next movement of crypto market accurately. These people making a decent living from trading you mentioned must have another sources of income to ensure their finances continue doing well in case trading fail along the way.