I agree with Grégoire and I can understand that change can be difficult across the team.
If you need a temp solution during the change management phase, you can build a smart list in Marketo where "MQL date" (within last 30 days) + "Data Value Change" (Product Line: ProductName, Add constraint: Previous Value - ProductName). This will allow you to build smart lists for all product lines. Your counts in these lists are then added to the current MQL Product Line counts (previous values + current values) to determine MQLs for the month per product line. I hope that makes sense. Expect a lot of duplication of people across multiple lists, though.
To Gregoire's point, by saying an MQL is attributed to a specific event, you then lose the justification for content creation, video spend, and ppc - it paints a picture that other influences aren't worth the budget. Marketing would be best (imho) considered a full spectrum activity that brings someone to MQL based on lead score activity. Your lead scoring is already set up to handle this.
thank you Willow Allen and Grégoire Michel! I do agree with you both. It will take some educating upper level management. I'll take on that challenge.
Fully agreed on that. There are 2 things that really matter with regards to Marketing measurement: