We have a consultant that built this before i was hired and my understanding is it was due to performance issues using Power BI (and maybe a bit of the comfort feeling of excel with the business). They still use the dashboards for higher level analysis and KPI's.
Get that company on board with Power BI. I've used Power BI extensively in a small/medium sized business environment and have seen FAR MORE STABILITY in Power BI than anything in Excel using Power Query and Power Pivot. I am constantly moving solutions I've created in Excel to Power BI so it is automated and stable.
THen use the Analyze in Excel connector for those solutions that must have Excel as an end-point. Like printing data to PDF - something Power Pivot and Excel are better at than Power BI is.
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DAX is for Analysis. Power Query is for Data Modeling