I think this would be useful in theory, similar to awardwallet which I find fantastic for tracking all of my family accounts.

But you need to think a bit bigger than just keeping credit cards active (to be fair it sounds like you are).

For many of us with established histories, the issue is not keeping cards active but rather cancelling cards to avoid paying AFs. So a good tracker would cover expiration dates, last usage, etc. In addition I would recommend columns for last 4 digits, CVN and expiration dates. I often cross reference such info when prompted by a merchant where I have stored card info. I currently use a spreadsheet and a note to do this but it is highly manual and inefficient process.

Finally I would recommend an AF rather than a monthly fee, similar to awardwallet. Maybe start with a free trial that auto-converts to an AF if people don't cancel.