1. it doesnt mean anything unless you say FOR WHOM it is a waste of time.
2. increasing numbers of people seem to be deciding that FOR THEM it is a waste of time, and so they are getting out of the field, or they are not writing so many research papers as they did. (However on arxiv I see a growing number of string papers by people at Beijing Normal and other large Chinese universities. These people do not think string is a waste of time for them and they are responsible for an increasing fraction of the research postings.)
3. your statement does not have a clear meaning unless you specify a waste of time AS WHAT. I think a lot of people would agree that string theory can lead to ideas and results that are interesting AS MATHEMATICS.
4. your statement would not apply to a mathematically gifted young person who goes into string and discovers something interesting and valuable AS MATHEMATICS. You cannot say that such a person is wasting their time!
What gives mathematics intrinsic WORTH is the interest it evokes from other mathematicians. It does not need to be a fundamental testable model of nature.
5. however your message, suitably qualified and restricted, is a very helpful one to have expressed---and voicing it actually DOES STRING THEORISTS A FAVOR
by increasing the pressure on them to arrive at a nonperturbative background independent formulation that makes falsifiable predictions. This is the only way to be sure that string theorizing is not a waste of time AS PHYSICS.