I for one would like to see a OSX-only photo editing suite. If it's anything like Pixelmator (no UI/UX compromises for the sake of cross-platform, insanely fast hardware-accelerated-everything) it'd be a real treat.
Beyond the obvious editing features you've got colorspace handling issues that most image editors don't bother with (Pixelmator actually handles this fairly well already), the need for a very robust plugin system, non-destructive editing, cataloging features (including image stacks), and a huge amount of largely unnoticed work that goes into simply keeping up with RAW formats for new cameras as well as profiles (for automatic CA/distortion fixing) for new camera/lens combinations.
Could this be largely mitigated by only working with DNG files and letting Adobe handle the conversion? Their RAW->DNG converter is free after all.
It is hard to describe how valuable that workflow simplicity is but it is a real thing; if I had to RAW process and edit all my photos using DPP (Canon's RAW editor, for my 70D), or Sony's RAW editor (for my A7) or even RawTherapee (an open source RAW processor that actually does a good job of supporting lots of RAW formats), I could do so, but yet I pay Adobe $10 a month to give me a really nice workflow for all of this.
It certainly wouldn't be impossible for a Pixelmator-like competitor to emerge for Lightroom, it'd just be a lot of work for a small team; perhaps more work than it would seem at first blush since there's a lot of complexity hiding behind a relatively simple UI there.
In other words, raw (as opposed to linear) DNG is basically a container format for sensor data and related metadata. So even assuming Adobe's DNG converter perfectly supports your camera, you still need code to handle its underlying sensor data format (your Bayer demosaicing code won't be much help in processing data from, e.g., a Foveon sensor) and whatever mandatory image correction it requires (e.g., many cameras and lenses rely on software correction driven by camera- or lens-supplied metadata, and will yield heavily distorted images if this correction is skipped).
In other words, until camera development stagnates, raw processing is a moving target by its very nature.
With that said, AFAIK, Aperture uses public OS X APIs for raw support, so potential Aperture replacements on OS X can get up-to-date raw support "for free". Given that Photos will support camera raw, this announcement gives me no reason to believe Apple won't continue to add support for new cameras in future OS X updates. Which is not to say Apple will support as many cameras as Adobe, of course — I'm pretty sure they already don't — or that support for new high-end (think $20K+ medium-format, not Nikon DSLR) camera support will decline now that no first-party professional software requires it.
I think (generally speaking, for most people) that Linear DNG using the Adobe DNG conversion method asked about is probably sufficient, if inconvenient. Most people I know who shoot RAW do it for the high-bit-precision dynamic range and white balance adjustment capability, both of which Linear DNG preserves.
But there are very valid reasons not to want to settle for partial-baked DNGs. Personally I was converting my CR2s to DNGs at Lightroom import for a couple of years (since DNGs compress and take up much less disk space) but regretted that when Canon's DPP DLO got really useful and I was unable to easily get those DNGs back into DPP as RAW.
Now I'm a big fan of saving the actual camera RAW file as the 'digital negative' even if that means the files are bigger and the format is more likely to be 'obsolete' in the future (there will always be older versions of Adobe DNG converter and dcraw to use if that is ever a real issue for me).
Photo editing is bitmap editing, and Pixelmator is well positioned to implement the non-destructive property.
By virtue of being OSX-only, they can abstract out the trickiness of RAW conversion (something Adobe must heavily invest in by virtue of being cross-platform), and rely on Apple's very good implementations instead. (side note: Apple is sometimes faster to implement RAW demosaicing support than Adobe itself).
This simplifies the problem to purely that of bitmap editing. It's important to note that unlike Lightroom, Pixelmator is already hardware accelerated from top to bottom, this allows real-time rendering of the entire edit stack on top of the unaltered original data, potentially giving us much more responsive feedback as users (sliding a slider and having it respond with much less lag than Lightroom, for example).