I, like many, can appreciate the amount of work that has already gone into creating Affinity Publisher Beta and I'm sure they will continue their work on the .indd import. In the meantime after running a few tests (both simple and complicated) I too have my workaround when need arises as @David Simpson has already stated in order to open an already finished InDesign document for further simple editing when necessary. Simply exporting the .indd into a PDF format imports quite surprisingly tidy into AP with editing possibilities that are fine for my needs.
As well, exporting from InDesign into an .eps file imports nicely although the text comes in as shapes so is not editable. Between both ways, this works for simple edits on an already finished .indd document. I wouldn't be switching to AP in the middle of a ID project anyways, so the finished .indd versions -> PDF is fine until AP gets the update for importing and hopefully exporting to .indd or at least .idml formats.
I understand a lot of the criticism at this point, yet still think Affinity deserves a good round of applause for taking on such a giant and I'm confident they will listen to their community (which personally I feel Adobe does not) and will produce their updates. I already like AP and am well excited to see where it goes.