This isn't just an opinion certain voters have, it's the truth. A vote for anyone other than one of the two parties with the best odds of winning is a "wasted vote" in the sense that when the third party candidate voted for doesn't win (and they almost certainly wont) that vote can't go to support anyone else. Our voting system makes that true.
The only way a third party could ever win in under our current system would be if they managed to get a massive majority of the voters to vote for them, and a massive majority of the voters knew that the third part was going to be getting a massive majority of the votes before anyone voted.
It's extremely unlikely that a third party could gain that much support. Especially because elections in the US tend to be pretty close. The best any president has ever managed was something like 60% of the popular vote.
Trump basically did just that and Bernie Sanders came close.
Decreased voter participation would signal a growing lack of faith or trust in the two major parties. That can open doors for fund raising for third parties, because they now can point to a percent of the population that historically did vote and stopped. Similarly, it would open doors for gaining support and votes because they can make the same argument to voters - 20% (or whatever the decrease is) of voters gave up on both parties. Join our party if you lost faith in the others, and in a 3 way race that puts us very much in the hunt for fundamentally shaking up the system we have today.