Honestly a FREE and simple web service for managing subscriptions sounds like a great idea. It could sent out emails every month saying how much you're paying for all your subscriptions, for how long you've been subbed to them, etc.

If someone builds this, please do not ask for yet another subscription. At least make it self-hostable and charge for the hosted one.

I think it already exists (and yes it's a subscription based software haha). I think it's called rocketmoney?