It's actually much more similar to how they kept using Majel Barrett as the voice of the computer across multiple series.
And it makes sense in-universe for The Doctor to be 800 years old and still working; it's not like they're contriving a way for Harry Kim or Phlox to be there, much as we may want to see them again.
That's a very good point, I only thought of the good doctor as a character, not a replicable computer program (despite us seeing several in VOY). It still feel slike fan service, but seeing him is at least much more plausible than running into Riker at every turn.
I found it interesting that in recent articles quoting Kate Mulgrew on her conditions for Janeway to return in live action, the thing she most stressed was that she had told Alex Kurtzman that the quality of the writing would have to be meticulous.
She’s very happy with the writing for Janeway in Prodigy but sounds like she needs to be convinced that it would be the same in live action.