It makes me wonder if sentient systems would treat humans the same way humans have treated animals. A more likely scenario: Power gap between a select few and the rest widens massively. (There's probably a few people who are selfless and want the greater good of the society)
Sure there are. Whether or not that remains true forever remains to be seen, but currently, most jobs can't be completely automated.
But I think the answer to "what is the purpose of humans" is the same as it has always been: our purpose is whatever we decide it is.