People that invest in Bitcoin just for a short term do not know what they are missing by selling too early. People consider profits making to be the main goal of buying Bitcoin therefore they get comfortable with any profits that they made provided they did not lose but that mindset is making people to lose alot of potential gains that they would have made if they they did not sell early. People that sold in the highest price of 2021 bull market and did not buy back because of procrastination or other reasons, did not see the price get to $100,000 that just happened few days ago. They would be regretting but there is not much that they can do again. We have to be willing to keep hodling until we get a very good profit that we will be very comfortable for us.
They will regret it if they don't prepare from a long time ago especially if they see Bitcoin price now. If they want to invest Bitcoin right now, they can still make a profit although that will depend on how much initial money they will use. Maybe they can use DCA now as Bitcoin price is not yet surge to the highest price but still, it is not easy to buy at the time now because the Bitcoin price itself is already in a high price.
If we can suggest to them, they can still buy it and not give up although the price is already in a high price. If there is a will, they will still invest their money in Bitcoin and wait for the bull run to come and sell their Bitcoin to get the profit. After that, they can start from the beginning as Bitcoin will be on the new cycle.