Enabling customer self administration/configuration of a "B2B SaaS" system mandates some form of interaction. I would be surprised at how many would not expect "touching a browser" to do so.
> A vast majority of the hours I've spent building web sites & applications has been devoted to administrative-style UI/UX wherein we are ultimately giving the admin a way to mutate fields in a database somewhere such that the application behaves to the customer's expectations.
If there is no validation of information given nor system rules enforcement, then I would question the correctness of said sites/applications.
> In many situations, it is clearly 100x faster/easier/less bullshit to send the business a template of the configuration (Excel files) and then load+merge their results directly into the same SQL tables.
This approach obviates the value added by a system enforcing core software engineering concepts such as fitness of purpose, error detection/reporting, business workflows, and/or role-based access control.
In short, while letting customers send Excel files "and then load+merge their results directly into the same SQL tables" might sound nice, this does not scale and will certainly result in a failure state at some point.