It doesn't need to be "a DAW", it just needs to be "the parts of a DAW that lets you make the sheet music sound good". If they pull that off, which to date literally no one has, then I couldn't care less if it wasn't suitable for general music production without the sheet music side in the slightest. Need to be try to be FL Studio, or Ableton, or Logic Pro, or Cubase, or Avid Pro, or Studio One, or Reaper, or... Do what they don't, because they're not going to.
If MuseScore will let me use Spitfire's BBC Symphony Orchestra and gives me DAW-style control over how my sheet music renders to audio with the finesse that score alone doesn't allow for, then that would be something we've never been able to do, and it's about time someone did it.
Genuinely curious, why is that better than just having good midi i/o? I would think almost anyone doing composition and notation, who has expensive, realistic orchestral libraries to play things back on, already has a DAW of some type. And the type of full control you're describing sounds like it could really include a lot. You'd need features like a recording timeline and playback control, parameter automation, advanced routing options, return tracks, side chain inputs, plugin latency compensation, ability to freeze or record plugin output to save cpu resources. Pretty much everything a daw has.
I've long been a fan of PreSonus's Notion (and I say that as someone who has a lot of experience with Sibelius, Finale, and Dorico). It's one of the few products out there that pays attention to the "parts of a DAW that let you make sheet music sound good," and I (personally) find the UI easier to use than most. I won't say it's the best thing out there if you need beautifully engraved parts for a large orchestral work and you need super high levels of organization, but I love it for composing.
I do agree, though, that no scoring program really let's you have the level of control over the way sheet music is rendered at the level that Spitfire or VSL allow. There's always some kind of disconnect, and I always end up tinkering with things in a full-fledge DAW in the end.
If MuseScore will let me use Spitfire's BBC Symphony Orchestra and gives me DAW-style control over how my sheet music renders to audio with the finesse that score alone doesn't allow for, then that would be something we've never been able to do, and it's about time someone did it.