Yes, I agree that it is likely not feasible. Both score editors are most likely tied in fairly tightly with Cubase internals, and Cubase has a very old codebase which increases the likelihood that it has weirdness in places. Changes to some of those internals would most likely break the score editor, if the score editor did not receive updates to match. Having to update both the score editors would mean twice the work.
A potential example is the expression maps system. Right after the new score editor came out, they mentioned an intention to enhance the expression maps feature in the future. It had then immediately occurred to me that most likely, the old score editor is a main reason why the expression maps system did not have any major enhancements for a long time. It seems as though the old score editor was tied in on a very deep level with the current expression maps system as it is designed, and so changes to the expression maps design would doubtless require major changes to the old score editor just to keep it working (without crashing everything). It also wouldn’t necessarily make sense for them to make expression-map-related enhancements to Cubase until the new score editor was in place and they didn’t have to worry about potentially breaking the old score editor anymore.
I’ve been a Dorico user now for about 7 or 8 years (and a Cubase user for 20 years). One thing I should mention here is that one of Dorico’s big strengths has always been how well-designed the automatic systems generally are in reducing the amount of effort it takes to get a really nice score and parts on the page. They save a huge amount of time in this front.
Yes, you do occasionally need manual adjustments. But in fact in Dorico there are also very often ways of controlling some of the automatic features in a better way, and this can significantly reduce if not eliminate the need for manual intervention.
With other score editors, you often have to use manual methods for everything, but in the case of Dorico, the automatic functions are so well designed and configurable and adaptable that in many cases you can write entire beautifully engraved scores and only do one or two manual adjustments, if any. I’m able to produce scores and parts that look nicer than what I could do with Finale or Sibelius with much less time with less manual effort. However, it is fairly common to need to use the controls over the automatic functions provided in Dorico to get to this point (and even then, still on occasion manual adjustments are needed).
People learning Dorico are typically taught to always leverage the automatic stuff first, by being shown how to exercise controls over the automatic behaviors, and only drilling down to manual adjustments if the more automated methods fail (which is actually fairly rare given the levels of control over the automated methods).
Some of those controls of the automatic functions are already there, and Cubase users have not necessarily learned how to use them yet, but some of them are not exposed yet in Cubase. But, in Dorico you can actually get a surprising amount done without the manual adjustments that you would need with other scoring packages, and I expect the same to be true of Cubase once they add more needed controls into the software.