The president has a generally negligible effect on the economy in the short-term
Yes, we have all listened to that episode of
Freakonomics. However, it is not unreasonable to think that 8 years' worth of domestic and foreign policy decisions will have a fairly significant impact upon economic direction. Most Presidents tend to steer a fairly moderate course most of the time with only things like wars effecting significant and fast economic swings, and in the case of the US that doesn't necessarily mean negative swings - at least in the short term. But it doesn't take too long for things like phoney trade wars with the world's largest exporter of consumer goods to have a noticeable impact so let's not play the "hey, nothing bad is really Trump's fault because Presidents don't impact what happens to the economy" card just because Dubner said so, especially when said President has been claiming all the credit for doing so in the first place.
