Is there a witty or general saying of indicating the act of taking over a business when a person, business or country skips an opportunity for general benevolence?
Examples:
Though this phrase might not have the negative connotation you're looking for, you can certainly say "If I don't sell weapons to country X someone else will fill the void", which is another way to say that the part of the business that someone might not opt to pursue (for good, bad or whatever reason) there will be someone to avail themselves to the open opportunity that might not have the same compunction about whether the business opportunity is moral.
The phrase you include yourself, "if I don't, someone/somebody else will", is indeed quite commonly used.
We might also talk of someone "filling the vacuum". This alludes to the idea that "nature abhors a vacuum", and hence suggests that somebody taking over that area is an inevitability.
(The related phrase "power vacuum" refers to the similar idea but in regards to political power; if someone loses control at some political level [national, international, local, within organised crime, within a particular organisation], it leaves a power vacuum that we can expect to be quickly filled by another).
Perhaps this quote
It is useless for the sheep to pass resolutions in favor of vegetarianism, while the wolf remains of a different opinion.
- William Ralph Inge
