The biggest reason I have seen that also resonates with me and is why I upgraded, is simply because the mDave is not perfect. For whatever reason, that comes off as blasphemy in this thread, which is just unfortunate as this applies to literally every single other piece of gear I have ever heard.
There are DACs that are better than the Dave. They are out there; and you see a lot of people that move on and don't come back. The Pacific alone is a big upgrade from a resolution, spacing and realism standpoint--something I totally did not expect. Would have never imagined, and I mean an immediate big wow factor difference. I personally wanted a new dac because the Dave can come off as a bit thin and analytical at times, and I also was hoping for a boost in bass performance. With the Pacific, the tone improved, with improved detail, but the bass performance and presentation seems pretty similar overall. (This is with headphones btw, I don't use the Dave with my speakers, but that may likely change).
That said, the Dave is still very, very good and the main issue I see is to keep the same level of detail and resolution or better, while improving in others, it does seem like you have to spend quite a bit more. That alone is an amazing testament to the Dave. And no one that owns one should ever worry about it not competing with other Dacs. But this idea that it doesn't get better than the Dave and anyone moving on is just on an upgrade chase is a bit ludicrous. FYI, I agonized about moving on from Dave for years. And I still haven't and will likely get the cap and LPS mods done and then re-compare w/the Lampi (which I suspect may move it ahead bass wise, but we'll see).