There has never been a concert in Lakewood. And by a concert, I mean an event where the main attraction is sitting and watching a singer perform. I don't expect anyone who didn't grow up in this environment to understand where they are coming from and it would be a waste of time to try and explain it. The stadium was only one concern mentioned in the letter.
I don't know what environment OP grew up in, but I'd be shocked if he has attended a concert in the last 30 years. I'm sure he understands the sentiment perfectly.
Having said that, there has been no concert at all? Even Uncle Moishy? Color me skeptical.
My point is, the definition of concert is fungible. The event doesn't advertise a concert, only an appearance by Benny Friedman at a Lag Baomer rally. If the rally is like the others I'm familiar with, it really is not a concert, but a gathering with Torah, Tefillah, Tzedakah, and yes, some musical entertainment. Even the reincarnated Great Parade in Crown Heights in 5770 separated the concert from the parade.
If the dissenters would come out and say, "don't send your kids, it's a Chabad event and we don't agree with their hashkafa, vchulu," that would be more honest and understandable to me than coming up with excuses like the stadium and "concerts" which will fall away under scrutiny.