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There is no guarantee that you will make a profit by investing in Bitcoin. The Bitcoin market is very volatile, there is never a guarantee that you will make a 100% profit by investing in Bitcoin. But the risk in long-term investment is much lower than in short-term investment. No one is ever able to predict, all we can do is guess. For example, by looking at the market demand for Bitcoin and the history of Bitcoin, we can guess that the price of Bitcoin will increase in the future. In the case of long-term investment, you do not have to worry about small ups and downs in the market.
There is no reason to go from one extreme to another - to suggest that since bitcoin's price is not guarantee, and perhaps nothing related to bitcoin can be exactly guaranteed, and so therefore we are just guessing.
I doubt that we are guessing, especially since bitcoin has a lot of quite interesting features, including that it paradigmatically changed how value and/or information is communicated (transmitted), and so there are a lot of ways that bitcoin is likely to be used in the future that are not even actually clear today. There is more behind bitcoin than a mere guessing, but a foundational interesting world-changing invention (discovery), but hey you can think about bitcoin however you like, even though referring to bitcoin as a guess causes me to believe that you likely have hardly any clue about what bitcoin is as compared with other assets and/or how bitcoin differs from other assets.
most newbies are really have this mindset of guessing bitcoin value potential, in addition to what you have said JJG Bitcoins decentralized nature and block chain technology have opened up new possibilities for secure, transparent and efficient transactions which have been the best asset that have ever had to mankind in history.