I don't think anyone, by now, really believes that using MCT as a matter of course will not reduce engine life. Too many however seem to think there is a time limit on the use of MCT. Engine manufacturers certify an engine to operate within certain parameters. They have ascertained that MCT does not cause "damage" to the engine and therefore do not impose any time constraint on its use. However, they do acknowledge that it is preferable to use lesser thrust settings, where MCT is not required to sustain safe flight, to extend engine life beyond what they believe to be the reasonable design life of the engine. Don't really think there is much more to said on this matter.