A spreadsheet can be an effective business tool. The case for an application becomes clearer when several people need controlled access, a reliable change history or a workflow the file cannot support comfortably.
Find the problem the file cannot solve
Staff may overwrite one another, maintain conflicting copies or struggle to find the current status. A formula may contain a business rule nobody feels able to change. Those are more useful clues than the number of rows alone.
Inventory the sheets, formulas, hidden columns and manual corrections. Ask how the team uses the file through a complete task. Some formulas may be workarounds for missing information elsewhere.
Preserve the useful behavior
A new application should keep the flexibility people need while making important decisions explicit. Decide which fields can be edited, who owns each stage and how exceptional work is handled.
Test a small prototype with the people using the spreadsheet. A polished form that takes longer to complete may not be an improvement.
Bring a redacted sample and an ordinary journey. The first project may be a focused tool, an improved spreadsheet or a staged replacement. The aim is to solve the working problem and agree on the ongoing responsibility the application creates.