I also had this idea before, and actually used several tools like this.

The main problem, is that I keep having new subscriptions, old ones are cancelled or changed, and then I have to keep maintaining this list, which ends up being out-dated very quickly.

Some tools use your connected bank account to check for subscriptions, but that is too privacy-invasive for me (and even illegal in many countries).

I am not sure what the solution can be in today's world.