People have pulled off 51% attacks on altcoins that were arguably popular at the time, most notably Bitcoin Gold. However, note that a successful 51% attack doesn't allow the attacker to steal arbitrary balances. What it mainly allows them to do is to undo their own spending, which they can exploit to commit fraud on exchanges in the short term.
This is the correct threat of a 51% attack, although if the currency is unpopular enough, had little use and everybody stopped mining, you could theoretically fork from the genesis block and replace the whole chain as soon as your fork becomes longer than the original chain.
There is little to gain from attacking a system _that_ unpopular, but you could "take" everybody's coins in that scenario.
