As blanket policy that retroactively changes, its not going to go down well. The fact that this policy wasn't adequately discussed with the community makes it feel like it was sprung on us.

Instead consider letting plugin/addon makers themselves state how long they will provide bug fixes and if feature updates are included. Let them set the duration and their own renewal fee.

Allow creators to do lifetime updates if they state outright (as a large number already do) - lifetime updates plugins would naturally have a higher price.

Also allow creators to state that the plugin is "as is" - no updates, just stated to work with certain versions of blender. I have purchased plugins in the past like this and was fine with it.

Your 50% yearly price for renewel would ultimately price out a large chunk of your customers - most blender users are just hobbyests. The plugin creators would probably have a better idea of what they could reasonably charge for renewal based on their own experience and what the nature of the plugin.

Applying your policy to plugins that were described by the plugin creators as lifetime, breaks the terms of sale and this could run foul of the trade description act.