I think Michael Piller's intention was that assimilation is permanent but Picard had only been assimilated for a few days and the process was not complete. The Borg also didn't completely destroy Picard's personality in order to allow him to function as the spokesperson for the Borg making it easier to recover Picard. Any other victim would have had their entire personality erased once all relevant knowledge was assimilated into the Hive Mind.
And that was pretty much the impression through all of TNG. Hugh wasn't able to be restored nor were any of the Borg that were part of Lore's cult. None of those Borg separated from the Collective remembered their lives or names before assimilation. If they had they wouldn't have been so confused about how to behave as individuals nor would they have chosen to stay with Lore instead of returning to their families. And Dr Crusher never once brought up removing the implants and restoring Hugh or the rogue Borg like she did with Picard suggesting assimilation is pretty much permanent.
VOY was where the writers introduced the idea that assimilated individuals could regain their individualities/memories and that their bodies could be restored. Realistically Seven, who was assimilated as a pre-pubescent child and was placed in an assimilation chamber, would not have had her human systems start to regenerate and reject her Borg technology but it was done so that the writers could get her out of the Borg costume and into a more flattering catsuit and to get Jeri Ryan out of that hideous Borg make-up and let her normal face be visible to the audience.
It only continued from there with the Borg kids, the "Survival Instinct" trio etc.
