Under plurality, if you don’t vote for a front-runner, your vote is “thrown away” immediately. RCV actually reduces the risk your vote is wasted by giving you back-up choices, so your vote can continue to count when your favorite has too little support.
Under RCV, voters are free to rank as many or as few candidates as they like. When a voter stops ranking, they are saying that they have no preference between the remaining candidates; i.e. if only the remaining unranked candidates were on a plurality ballot, they would not vote in that race. A voter’s ballot becomes exhausted only when the voter ceases to have a preference among the remaining candidates. Their vote isn’t being thrown away; rather the voter is deliberately choosing to no longer have a say should all their higher preferences be eliminated.