The manuscript and project model are primary. AI is an optional tool attached to them.
An external model is a dependency
Models, APIs, prices and policies change. A writer should not lose the core workflow because one external service changes or is unavailable.
Many writing tasks do not need AI
Moving a chapter, comparing versions, checking project structure, reading the manuscript, managing comments and exporting a book are deterministic tasks. Adding a model would not automatically improve them.
Optional AI preserves choice
Some writers want AI for alternatives, condensation or editorial questions. Others do not want model-assisted text at all. Both workflows should remain first-class.
This also keeps the product honest
B-Maker should not invent AI steps simply to make the feature visible. The model appears when there is a bounded problem where probabilistic assistance is useful.