It's not called a monopoly. A monopoly is a single provider of a particular service or product allowing the supplier to abuse their position.
These farmers do not have to buy John Deere. If they don't like being tied in, they shouldn't have bought the equipment in the first place and chosen a rival then one of two things happen 1) John Deere realise they have been caught with their pants down and change their policy to win customers back 2) Farmers realise there is a new normal where the equipment is simply too complicated to allow them to tinker with it (or at least without losing any manufacturers guarantee).
These farmers do not have to buy John Deere. If they don't like being tied in, they shouldn't have bought the equipment in the first place and chosen a rival then one of two things happen 1) John Deere realise they have been caught with their pants down and change their policy to win customers back 2) Farmers realise there is a new normal where the equipment is simply too complicated to allow them to tinker with it (or at least without losing any manufacturers guarantee).