Thanks for taking the time to share this idea.

The PayPal Payments module provides access to credit card-like controls which clients and admins can use to manage their payments:
- Clients can delete their PayPal account from the list of Payment Methods via the corresponding page in the client area . If they did this immediately after paying, you wouldn't be able to automatically charge them for renewal: https://docs.whmcs.com/payments/pay-methods/#in-the-client-area
- As an admin you can toggle on the "Disable CC Processing" option on their Profile if a particular Client doesn't wish their account to be charged automatically: https://docs.whmcs.com/clients/client-profile/profile-tab/#additional-client-settings

Linking of the PayPal account is really beneficial to merchants and your clients; it reduces renewal friction to help decrease involuntary churn for you, makes it easier for clients to upgrade and buy new services without having to re-enter their PayPal account details and reduces cart abandonment. Faster checkout = higher conversion rate for you.