What would even be a reason for someone who wants to hold Bitcoin for a very long period to take a loan to invest in Bitcoin? When they have a long period of time, they could accumulate Bitcoin gradually by making use of DCA (Dollar Cost Averaging).

I would just say that he or she is not truly interested in holding Bitcoin for the long term. People who take such risks are often those eager to make quick profits. Their aim is to sell within a short period, and they are considered traders. This is a very bad idea because even trading is a big risk. Trying to take out a loan to outsmart the market is the worst thing such a trader can do.

It is always better for any investor to invest only what they have and can afford to lose. Some people lose money in Bitcoin because of their eagerness to make quick profits in Bitcoin.
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I agree with you that any investors should only invest what the have or afford in times of lost profit, but what many investors always do is to have the mindsets that are going to make quick money in Bitcoin, and that will make them to put their family problems on Bitcoin, with such investors that have mindsets of making quick money on Bitcoin and selling, they can't never buy dip and hodl for a long term investment, rather they  will make use of short term investment, so for an investors to buy dip and hodl for a long term, is to remove his or her carrying the family problems in place of Bitcoin, all in the name of making money, you can only enjoy this buy dip and hodl when you buy and invest in a long term investment to accumulate Bitcoin gradually by making use of dollar cost averaging.