If arc is continuing by momentum, it's a very different kind of momentum than the momentum languages like Java, Python and Perl survive on. They have momentum in that they have many large and complex libraries and programs written in them which are being actively used to the point where rewriting said software in another language is no small task; and hence most people are interested, not in making a new and better language, but in making the old language better.
Arc has none of this. The reason why it continues is because the people using it are enamored of it for its own sake, not because they depend on it. This is both a blessing and a curse. A blessing, because it means Arc is free to change and improve; a curse, because it means Arc is incomplete and a moving target.