let's take for instance now, I a big dip occured in bitcoin, and you are yet to be paid from your work place, and you really don't want to miss out buying that dip, buddy there is nothing bad in collecting loan to invest in bitcoin, to buy that dip and hold provided you have a source or other sources of paying back the loan without tempering with your bitcoin investment. And it's not only necessary for you to take loan only when it only has to with emergencies, you can also take loan to invest in bitcoin provided you other ways of paying of the depth without touching your holding.
In the example you gave, no one will stop you from investing by taking a loan. Because you know that you can repay the entire loan within a few days. This is a common situation for all long-term investors. Those who are real investors prepare themselves for all situations in advance. If for some reason they face a financial crisis, they try to get out of that situation even if they take a loan. And in a very short time, due to a proper money management, they return to their previous position. Still, they do not sell their investment.
But if someone starts investing by taking a loan without understanding and does not have a financial backup for the next situation after taking that loan, he will never be able to maintain his investment. To deal with the current financial crisis, he will have to sell his investment. So if you do not have a proper money management, you should never invest by taking a loan. No matter how deep the market goes.