It would be great if we have better interface for migration. But the challenge is that migration scripts are far from complete thus it’s not helpful for building an interface. While the execution of scripts, you have to write more codes for converting posts, import more fields and even read error messages when it aborts in the middle of process. That’s why it’s meant to be for developers now.
There are a lot of developers contributing to scripts. Discourse has many import scripts now. I’d say it’s quite an effort even they are far from complete to be used with a good UI. Anyway, paying a developer to migrate sites and asking them to submit a PR definitely leads a way with the proposed feature. I would hope that time comes soon.