Say you have multiple product lines like iOS-1month, iOS-1year, iOS-1year-premium, Android-1month, web-5years, etc. And you have historical numbers for new users and retention for each product line.

I looked at using Prophet for this, but instead I made a giant spreadsheet and modeled my assumptions around seasonality, retention, and new user growth. I wasn't totally sure, but it seemed like Prophet wasn't intended for my use case, where my assumptions can be stated very explicitly, and I want to twiddle the knobs to look at various scenarios.