Inspired by reading King Leopold's Ghost, which compares the Belgian genocide in late 1800's-early 1900's Congo to the Soviet Gulags, and then reading Imperial Reckoning, which compares the British torture camps in 1950's Kenya to the Soviet Gulags, and also by reading Golden Gulag, which compares mass incarceration in California to, you guessed it...
Hmm... Maybe for the workers in the synthetic rubber factories. But unless an equivalency in the productivity of plantation-grown rubber, IDK about that.
Remember that an increase in the average productivity of a sector is a decline in it's average demand for labor-hours; that is, the proportion of the labor-value that gets paid to the worker for the same hour of work is reduced. If you are using a non-standard process that takes more labor-hours to create the same amount of product then you going to be operating at a relative loss to your competitors even if you're paying them at the new proportional wage-rate. That means in order to be competitive, you have to pay them an even lower portion of the total product of the process than what the new, more-"efficient" method of production provides the worker. That or you need to intensify the pace of the labor significantly.
Source for this: Marx' bit about the introduction of the Spinning Jenny to England in Capital Vol. 1
So in other words, natural rubber farmers get fucked even worse because they have to compete with cheaper synthetic rubber?
That's the idea, yes.