I only see a benefit to developers for this, but from a user prospective going with another system is a downgrade.
Assuming google works like Apple (correct me if I am wrong), disabling a subscription should be able to happen from a central location with a click or 2.
If I instead go with the billing through a company not only do they now have my credit card information, but I have to go through them to cancel. Meaning they can send me through screen after screen trying to convince me to stay (dark pattern) or even worse forcing me to call to cancel.
As a user, if you want to offer this fine. But as long as the ability to subscribe through Google or Apple is not removed I will be fine. But if this starts a trend of more and more apps having their own billing that then uses dark patterns to keep me subscribed... I will just end up spending less money on subscriptions than I currently do, and I have quite a few subscriptions.