I don't believe that Trump's absence will really have that great of an effect. Since 1924, ever president has attended that dinner, at least once. Some .. it was solely once. Trump's been there many times in the past. He probably won't break tradition and he will attend one, sometime in the future.
Bloomberg and others aren't throwing their normal big bashes .. and those things, supposedly, were quite the extravaganzas ... Everyone that was anyone had to be seen.
If there's a big drop in attendance .. more than likely .. that's the cause.