This is typical. A few good polls and suddenly D's on Atlas starts celebrating the inevitable 2020 victory. Sure, there has been a discernible tick down in Trumps numbers over the last couple of weeks. But whatever might have caused it (McCain, Cohen, WH insider, Woodward, etc), there is no particular reason to believe that THIS was the final straw that broke Trump. If the past is any predictor of the future, Trump is likely to bounce back. As long as the economy is strong in 2020 (it may or may not be), Trump will be at least competitive.
His approval rating in the RCP average hasn't reached 45% since February of last year. When did he "bounce back"? His approval ratings have been varying degrees of poor since his first couple months in office.