I don't believe Pratchett signed up for cryonics, but if he had, what good argument could there be for not letting him commit assisted suicide in the way that best prepared his body for cryopreservation, followed by immediate cryopreservation?
If medical science admittedly can't do anything for these people other than offering palliative care, it seems like from a sanctity of human life perspective, cryopreservation under optimal conditions at least offers a chance to preserve the life that assisted suicide opponents hold to be so sacred. Maybe rather than repealing laws against assisted suicide, we could work towards getting exemptions for cryopreservation?
Would be bad for cryonics. Assisted suicide opponents would scream "Cryonics gives people false hope so they can kill themselves!". People with bad but solvable problems would be more likely to choose suicide + cryo, as opposed to solving their problems (also as opposed to regular suicide, so it's unsure which is sadder - but the opposition would see the extra expected deaths more than the prevented ones).
At the very least, opposition to assisted suicide + cryo for the terminally ill seems like it would be very hard to defend.