Excellent point. Yes, I would actually prefer to be charged a yearly fee for serious plugins that I am using in key ways.
As Chris Lema has blogged, the challenge is that the 'one time fee' business model can often become unsustainable because the new sales might drop off and existing customers are still driving for support and feature expansion.
For serious core plugins, I want the developer to do well in the service they are providing all of us. It is in all our best interests that they both financially succeed and feel successful/enjoy their work.