"In the long run we are all dead", said British economist John Maynard Keynes. The converse also contains a grain of truth: we live in the short-term. There is no reason to think that a gain made over a brief period is any worse, economically or morally, than one made over a longer time frame.
Keynes was not arguing in favour of short-term speculative gains when he made his famous observation - he was arguing in favour of governments and financial authorities taking prompt action rather than letting economic matters slide. He had no problem making short-term personal gains speculating on the stock markets, however.