So, what if Justinian, after conquering North Africa, Sicily, Baleares and Sardinia- decided that he will not try to conquer Hispania or Italy?
And instead he decides to conquer whole North Africa ( western part )...
I doubt this would happen: although there was only a thread on this matter last week!
As for western North Africa, the region seems to have been politically semi-detached from the Roman Empire for most of its history. Certainly from Valentinian I onwards we see "Romano-Moorish kingdoms" emerging in the region, which seem to have been the setting up of hereditary lines of Roman governors to run a province with very little in the way of direct supervision from the centre. A similar phenomenon can be seen in Britain around Hadrian's wall, and may be at play in Syagrius' domain.
Why not?
Can't Justinian reverse the trend if he doesn't try to reconquer Italy or Hispania?