Speculation fills the gap before release because it's obviously filled with hype. At the beginning of speculation, there is a "yet to be determined" amount of new characters. This time started with Inklings, Ridley, and then eventually K.Rool. This game specifically confirmed a lot of fan favorites.
Going into DLC...well, Rex is probably out. Spring Man is out. Waluigi, Isaac, and Skull Kid are out. Most fan wants at this point are off the table, and DLC, as many have said, will probably get leaked beforehand.
Not to mention we've only got 5 characters to talk about, and speculation this time was entirely guided by leaks from beginning to end, making the endless cycle of "this was leaked, therefore your ideas can't happen".
Like, the question of, "Hey, why do you think speculation will drop off or be boring guys?" is completely ignoring the fact that we speculate because we're excited to play the damn game. If you're really wondering, this is exactly where the joke of, "Do you guys actually play this game?" or "I buy the game to stare at the character select screen" come in.
It's like people like talking about the game, but the idea of playing? Reprehensible! Blasphemy!