Apart from buying bitcoin during bull market, I think buying any other time can be profitable. The bitcoin market has trend, especially to the long time investors.It is always advisable to buy bitcoin when the price is low , so that when the price rises you will be able to make some profit.
When you buy your bitcoin when the market is already roaring ,it can make you loose money. Some people would continue buying bitcoin any time the price dips, and following market news will go a long way because you will always be informed when to buy, more especially when the market is in deep red.
You've got the core of a good investment approach: buying in fear and selling in greed. It's is impossible to time the absolute bottom and history has it that accumulating during bear markets and deep corrections ("deep red") rewards long-term holders. This dollar-cost averaging strategy softens out volatility and avoids the serious danger of buying at great top. Information is key, but discipline - buying when others are dreading - can truly build wealth in Bitcoin, using every market cycles advantageously.
Buying during the decline and selling after the decline is over will never be right. Because an investor should always keep his investment in mind for the long term. As you said, an investor loses more by doing so. Because we can never tell what will happen in the future. There are many businesses who buy with a lot of money during the decline and after a while after the decline when they see that the market has fallen again and their investment is losing money, they panic and sell their investment and they lose a lot of money. This should never be done, one should always continue to invest in a long term and should continue to buy continuously.
If a person waits for the decline, then he can miss many buying opportunities. Many times it is seen that the decline does not happen according to their expectations and then they regret a lot why they did not buy then. So this should never be done, a person should always invest in a long term and should continue to buy continuously by adopting the DCA method