Single Source of Truth? It Doesn't Exist
The first data analyst joins a startup. He sets up a data warehouse, ingestion pipelines, builds dashboards, and declares that from here on this will be the company's 'single source of truth': all reporting shall take place through this 'single pane of glass'.
Everyone nods along.
A month later, the VP of Sales shares some pipeline numbers. Our data analyst is bewildered: these numbers do not match what's coming from the data warehouse. He contacts the VP of Sales: 'I'm looking at my dashboard but don't see the same numbers you're reporting. Are you looking at something else?'. The VP of Sales responds with 'Huh? Your dashboard? My team just made a report in Salesforce. That's where we get all our numbers from.'
So much for a single source of truth.
The truth is out there
Every team has a home system. While the data team's home system is the data warehouse, other teams live in different systems. No salesperson leaps to the data warehouse to look up a customer. They go to Salesforce first. Similarly with finance teams and NetSuite, customer support and Zendesk, engineering teams and the production database, and so on.
There is no single source of truth. The truth is actually a pie divided into slices reflecting the company's org chart.
Implications for data teams
For data teams, the major implication of the above is to verify your assumptions with the teams you serve before spending time (i.e. money) on work. It does not feel great to spend months completing the ultimate finance dashboard, only to discover that your finance team ignores it because they work in a comfortable combination of NetSuite and Excel.
Talk to your coworkers and understand their goals. Some will involve problems you'll be able to help with (e.g. visualizing a complex funnel involving multiple teams and systems); others will be fine operating without you (for now!). But at least you'll know where to spend effort.
You may build the world's most pristine and accurate single-source-of-truth. But doing so in a vacuum is the same as having done nothing. Like with building any product, it's best to do your customer research first. A morsel of utility is often more impactful than a grand vision.