We do not know if Inter will beat Benfica to be fair, and we don't even know if Napoli will beat Milan as well obviously, however I agree that Napoli is favorites to win against Milan and everyone expects Inter to beat Benfica.
If you ask me Napoli will be in the finals, but they will beat Milan and then Benfica, because I believe that Inter hasn't been that good lately and they may end up losing to Benfica as well, still not a big trouble for Napoli and they should be easily beating any of those teams, all three look worse than Napoli so there is no trouble there. However the other side looks a lot more menacing, I think whoever comes from there may end up getting the title.
Not sure about Benfica - in the end they finished first in the group with Juventus and PSG.
On paper, probably in a pair of Napoli - Benfica, Napoli will be the favorite, but I would not put money on it. By the way, in general it will be one of the most tragic semi-finals if it happens - for both teams it is something like a chance "once in a hundred years" plus this season both teams are unusually bright and deserve to reach the final. But for some reason it seems to me that we will not see such a semi-final - in such cases, no team gets what it "deserves", the sport is often unfair and we can see Milan - Inter.