The file on somebody's
desktop is load-bearing
There is always a spreadsheet. It holds the regional mapping, or the customer tiering, or the targets, and it is maintained by one person who has been doing it for years. Most data projects plan around it and are surprised when that fails.
Why it exists
The spreadsheet is not there because nobody got around to building something better. It is there because the thing it holds changes in ways the source systems do not model: a reclassification agreed in a meeting, a target set by a person, a mapping that reflects how the business actually works rather than how the ERP was configured.
That information is real, it is used, and it exists nowhere else.
The two ways teams get this wrong
The first is to ignore it, model everything from the systems of record, and produce numbers that do not match what the business believes. The business concludes the platform is wrong, and in the way that matters they are right.
The second is to declare that it must be replaced with a proper application before the data work can proceed. That is a separate project, with its own budget and its own timeline, and the data work now waits on it.
Treating it as what it is
A spreadsheet in a known location, with an agreed shape, ingested on a schedule like any other source. It lands in the raw layer with a timestamp, so you know what the mapping was on any given date. It goes through the same quality gates, so a broken edit is caught before it reaches a report rather than after.
The person who maintains it carries on maintaining it. Nothing about their week changes, which is the main reason this approach actually works.
When to replace it
Once it is a governed source with checks around it, the urgency to replace it mostly disappears, which is a good outcome. Replace it when the sheet itself becomes the problem: several people editing it, versions circulating by email, or edits that need an approval trail.
Replacing it because a spreadsheet feels unprofessional is not a reason. It is doing a job, and it is doing it now.