Now don't get me wrong, I'm very optimistic, and have already bought/pre-ordered it (it's a gray area which to call it, because it unlocks some things immediately, but the bulk of it isn't available for a few months), but my optimism is mainly based on the Mario Kart 8 DLC, not what they've announced about the BotW DLC itself.
True, we don't know the specifics, but my point was that the core game doesn't feel like it was intentionally broken just to make way for DLC like we see far too often with games these days. Any new content is an expansion on what is currently a masterpiece.
Sure, we don't know how expansive an expansion it is, but I'd still argue that even a small release is an expansion at this point. I'm also giving them the benefit of the doubt considering how excellent the core game is.
In that sense, I think (from what we've heard), BotW's DLC is not really an expansion pack in the old sense.
BotW has been a system seller up to this point, but only because the hardware's supplies are fairly limited at this point. I don't expect the DLC's release will be a significant factor in the Switch's continued sales, unless their other first party titles end up having disappointing sales.