Every study so far (not funded by Uber/Lyft) shows that more people switch from transit than switch from private vehicles. Especially true in cities where the default is transit.
$2.75 for a subway ride in NYC or $4.00 for an Uber pool?
It depends. Which one is the faster trip? How much do you value personal space? Are you carrying stuff?
$2.75 vs $6.00 you are looking at the same factors, but for a lot of people, it gets harder and harder to justify the Uber.
But its a gradient. Not everyone makes the same choice