It is important you know that your investment into bitcoin has no guarantee, same reason you are advised to invest into it what you can afford to loose so that at worst, you would only loose 100% of what you invested into it.
Yea, there's no written document as to guarantee anyone of a given return in bitcoin but investors can see and feel it at all time. If bitcoin was a scam project or as risky as some traders make it appear, it could have lost all its credibility by now. For over 16 years now, real investors have studied bitcoin and mapped out a less greedy -high guaranteed system of investment (DCA approach using only discretionary income) but traders have refused to listen to that. This is the origin of this wrong orientation about investing what you can afford to loose.
Yes it is true that no one can guarantee profit by investing in Bitcoin. But if we look at the history of Bitcoin then Bitcoin has proved that holding Bitcoin in the long run can expect profit. Because the price of Bitcoin was very low in the past, but currently the price of Bitcoin is more than $100k dollars. Now very few people know about Bitcoin, in the future many more people will know about Bitcoin, which will increase the popularity of Bitcoin, due to which the price of Bitcoin will increase more in the future than in the present. So we can expect that investing in Bitcoin now can make good profits in the future. Now we cannot say that Bitcoin is 100% risk free, there is definitely some risk involved, but currently Bitcoin is very less risky. So we should invest according to our ability.
Looking at the history of Bitcoin, it is clear how Bitcoin behaves in the short term and the long term, Bitcoin is a volatile currency, so in the short term Bitcoin will naturally fluctuate constantly, but in the long term Bitcoin has always been valuable in history and Bitcoin will always be valuable in the long term in the future also.
Although we cannot guarantee the future, but in terms of probability and history and relevant analysis, Bitcoin will have a high probability of being valuable in the long term, Bitcoin is constantly making itself stronger, and people are understanding Bitcoin more and more deeply, so the demand is constantly increasing, so the price will definitely be positively affected.
All in all, Bitcoin should be bought at as low a price as possible while there is time, because there will not always be an opportunity to buy at such a low price, because Bitcoin is going to achieve a lot of great heights in the future.