A new class is half the selling point of an expansion, you don't go selling that separately unless you've given up on them.
Also the anniversary thing is a temporary event done up with VGA-esque shaders, its likely just an easter egg like the moo moo farm with no bearing on lore. The plan going forward rather looks like they want to gradually feed their daily players additional random content to enjoy rather than building campaigns for the one-and-done crowd that made up the millions of sales of the base game.
How does the Anniversary dungeon have no bearing on lore? Diablo 1 started the lore for the series, this dungeon is that game remade using D3, kinda like a nod to what started the series and a way for people to play the game if they never got the chance to play it. And the dungeon won't just be a temporary event, it will be something that happens once a year in January as it shows on the diablofans homepage in the section about blizzcon info. Hope I don't come across as rude, just genuinely curious how you think that this dungeon has no bearing on lore.