In the following situation, a product's layouts will be reset to the default layouts:
Create product under a product line
Set the layouts to inherit the product line's layouts
Edit the product to remove its parent product line (i.e. move it to the top of the hierarchy)
Preserve the previously used layouts as the new custom layouts for the product
Provide a "Where are my fields?" type of experience to help users understand why custom fields no longer show and give guidance on how to get them back
Thank you for the idea. Given the low volume of support for this idea, we do not have plans to make updates in this area at this time. We hope you an understand.