Bitcoin will help you to earn money, it helped a lot of people to earn money by just using technology. In my opinion bitcoin cannot help you to be rich, but it can help you to earn more money than normal work. Bitcoin is not a stable career it is because you are depending on the time when bitcoin is on a low price for you to buy some and waiting for the time that bitcoin will increase its price. It would take time for you to earn and sell bitcoin which means also that it would take time for you to earn money.
I think you are talking about people will buy bitcoin when the price low and sell when will be higher. I agree with you about that. This is one of the main strategy of making profit with bitcoin and it will take a time like you said, by the way, They can do trading which you can get profit in short time (I mean trading with others cryptocurrencys) which it is a great domain.
Everyone can trade and invest but the capacity of a particular person is important to consider, you cannot trade if you do not have the knowledge and skills as that will bring bigger risk which most likely will just make you lose overtime. Investing in the long term of what we called holding bitcoin may bring good benefits to you if you have a decent amount of money to invest, otherwise it will not take you where you want to be.