But even if not sensing pain at the same levels as more developed individuals, it is pain, or at least sensing (and observably reacting) to some degree that sort of stimuli.
And even if they didn't, that doesn't change what they are and shall become.
The "they are not advanced enough to feel like us" is an argument that had been used in the past to justify performing surgery without anesthesia on infants and the mentally disabled, and was used to justify the less than equal treatment of children, slaves, women, and still, to this day, animals, plants, etc.
We have been witnessing the growing empathy for alleged 'more' sentient animals for some years, or even for most in some circles, and while it is rare now, some may extend such to plants as well.
The sense of pain is just a processing of sensory input, it does not alter the fact that a living creature is alive.