There would be no motivation to build a plugin and only sell support. Why not just start a WordPress support company?

The question is also what is the motivation for writing good code and making it easy to use. There would more money when there more support tickets.

I believe it is possible to support someone else's code but I wonder how GPL Club handle bugs.

I think having a server side feature for a plugin is best.

If the plugin is free then it is impossible to sell updates as whenever there is update you can download the plugin again with the latest version.