I think that bar is off by 100%. I give it 3000 minimum.
Beyond the fact that it is clearly selling well after Platt leaves, It is also relevant that this is a small show. I am guessing that it has a low weekly nut and will be able to take advantage of that for years, a la Chicago. (Let's face it, Chicago would have closed years ago if it didn't have such a low breakeven). Why would Evan Hansen's be any more...it has a smaller cast, I assume the orchestra is probably similar sized, etc.
More importantly, the passion around this show exceeds anything I can ever recall, other than Hamilton and, possibly, A Chorus Line. (IMO, Lion King, Phantom, and Wicked are humungous hits, and legions have loved every one of those shows, but passion is a whole other dimension. Other than teenage girls for Wicked, I think the degree of passion around Evan Hansen far exceeds those shows).
I am 66 and the number of people I know who are in my age group and think it is one of the best shows they have ever seen is staggering. The millennials see themselves and their issues in DEH, even if they are not all outsiders to the extent that Evan is; older people see (in addition to a well written show with a great score) their children and challenges they faced when they were growing up. I could build a long, boring defense that that emotional pull is even more significant than for Millennials.
Re losing ground after Platt leaves, I say 'bologna'. Ben Platt is replaceable, just as Carol Channing was replaceable, and Zero Mostel was replaceable, and more recently Idina Menzel and Kristin Chenoweth in Wicked, etc. When stars of musicals have not been replaceable, it has generally been either because they were so famous that people couldn't fathom seeing it without them (which I am sure is one of Scott Rudin's worries since, somewhere along the way seeing Dolly became almost exclusively about seeing Bette Midler rather than seeing Hello Dolly); or the show was not any good (or at least not as good as the star) and the star was the only thing keeping it opened in the first place (e.g., The Boy From Oz or If/Then). (I am consciously excluding shows that were running out of steam when the stars left...the downward curve was already in place).