Both of the existing answers seem to be based on a mischaracterization of how migration and off-topic closures are related, particularly with things like
I would like to see Astronomy SE added to the list of options to migrate a question to, but not to make questions like this completely off-topic here. [...] I don't think having astronomy off-topic helps users.
I would prefer to migrate them rather than say they were off-topic.
Migration from one site to another requires two things:
- the question is likely on-topic at the target site
- the question is off-topic where it was posted.
We can't say "all of these questions should be migrated, but they're still on-topic here". Migration is, instead, a way of saying "this question doesn't fit here, but fortunately there's a separate site that can take it". This philosophy has informed a large number of decisions thus far, and it's baked into how the closure dialog is structured (with migration as an option under off-topic).
So, what options do we have?
- We can decide to keep the status quo as it is: astronomy questions are on-topic here. Posters can be pointed to astro.se as a potential better home for their questions, but closure is not appropriate.
- Alternatively, we can decide that a subset of astronomy questions (say, questions about amateur observational astronomy) are off-topic here and should be closed as off-topic ("I'm voting to close this question because it is not about physics"), and consistently flag closed astronomy questions so that moderators can migrate them.
What we cannot do, however, is set up a direct migration pathway to astro.se, because it is still in beta. I find this faintly ridiculous, given how long astro.se has been running (and therefore how little stock is left in arguments like "it would skew their site definition), but this is how things have stood for some time and SE doesn't seem to be in a rush to update things.
So, there is definitely a good call to make a choice as a community, between the two options above (and to set the scope for the latter), but the existing proposals don't really reflect the current reality, unfortunately.