no the idea is that in mimetreeparser there is no plugin loader, this should be in messageviewer. Because the plugins may be specific for a specific messageviewer, that's why the messageviewer needs to control this. For Kube as example the plugins would need different plugins, becuase tithey support a different set. That was the base idea of this BaseFactory to give full control ovr the plugins to the messageviewer.

The solution for that is adding a way to change the plugin search path (or add some other way of filtering the plugins by additional metadata), not to duplicate the entire plugin loading code. This becomes even more relevant once we add on-demand plugin loading, when this wont be a single function anymore.
Your approach would also mean we would never be able to use MTP stand-alone without MV, but I think e.g. our composer would actually only need MTP, not the full MV.