I guess the best way is for the indie developers to have a system and plan in place to support paid development, Tooling is already available to achieve this. For example, Pavilion has built an elaborate subscriptions system for discourse plugins. This is currently being used in custom wizard plugin subscriptions.
Similar integrations could be done by others to leverage the subscription ecosystem where users could subscribe for a small monthly fee to support the development and maintenance efforts for 3rd party plugins.
Ps: I am a part of Pavilion but opinions are my own.