Incubate lets you turn artifact tokens into Phyrexians, and the black-green draft archetype is all about creating bigger and bigger creatures (and incubate tokens)
Once Downerd goes to grave, theres a minimum of 3 types in the graveyard for Dichotomy, among anything else that may have gone to the graveyard
Lud0p13x didn't specify the playstyle for this deck
You get beats for later too, so you dont invest a card in the abilities, they are just nice additions
So far the Sphinx has only attracted a ton of kill spells when he hits the table so I'm not sure if he's worth keeping in.
If you're worried about Boros struggling to draw cards, fret not: cards like Losheel, Clockwork Scholar and Tocasia's Welcome draw cards on both our turns and opponents, and then we've got mass draw heavy hitters like Neyali, Suns' Vanguard and Commander Liara Portyr to really pop off