That seems a reasonable approach if the messianist "movement" is indeed waning rather than waxing. This is the kind of thing where you aren't likely to change people's minds. Better to let the passage of time take care of the problem.
This sounds ironically like the predictions made about the survival of Chassidism and Orthodoxy after WWII. Yeah, they're over, their influence is waning, soon there won't be any Chassidic Jews alive, anachronistic relics of a bygone age. Chabad proved them all wrong with their dedicated, idealistic approach, and I suspect that in the end the Messianists in Chabad will have the last laugh. As most Jews will admit in private, who cares who's right, just let Moshiach come already!
Furthermore, let's compare the kookiness of the Messianists to the Messianic kookiness of Judaism itself. A king will arise from the house of David. He will fight the battles of G-d and be victorious, gather the exiles, rebuild the Holy Temple on the site of the Al-aqsa mosque, and re-institute the Temple services including animal sacrifices. This is no Chabad invention, it's an integral part of Judaism and part of the 13 principals of faith codified by Maimonides. Those who are put off by the Messianism of Chabad need to check their Jewish sources and find whether it is Chabad that puts them off or something that is endemic to Judaism itself.
The problem is not messianism per se. The problem is with those who persist in calling Schneerson the messiah after his death. Jewish sources are pretty clearly opposed to the notion of a dead messiah.
Somewhere, there is another righteous man with all the qualifications of the Messiah. Right now we just don't know who he is, he himself may not even know. When G-D thinks the world is ready, the Messiah will be revealed to us.
Many Jews sang this song, even as they walked into the gas chambers of Auschwitz:
"I believe, with a perfect faith, in the coming of the Messiah. And even though he may delay, nevertheless I shall await his arrival, for he may come at any time."