I've heard this before, but haven't seen such points respond to the (to me) super obvious point that IP protected GMOs have plenty of competition (natural crops) so they can only generate profits on their improvement. One response I can think of is the optimal length of IP rights varies from field to field and is not necessarily long (link). Do you have a link to intelligent discussion of such issues?

I get the impression this objection is raised mainly because people react negatively to ownership over things which are not typically owned rather than a specific technical argument for why it's a bad idea. This has lead me not to pay much attention to such arguments, but I'd be interested to hear more detailed arguments if you think there is more substance there.