As Richard Baehr said in response to reports from  Axios  and  The Hill  that Paul Ryan will not run for re-election, "Paul Ryan packing it in tells you all you need to know about GOP chances to hold the House this year."   Certainly, that is the judgment of many Republican politicians and nearly all professional election-observers.   An opposition party "wave" would be the historic norm for a midterm following the election of a president from a party enjoying a congressional majority.

Because incumbents have a huge advantage, the  announced retirements  of several GOP House veterans and the prospect that more will follow could end up being a self-perpetuating prediction.

On the other hand, there has never been a presidency like that of Donald Trump, nor has there ever been an opposition effort so extra-legal and scorched-earth in nature as that conducted by the Deep State enemies of Trump.   Just as Special Counsel Robert Mueller is full of surprises, so may be the outcome of the I.G. report on the DOJ and eventual disclosure of the basis for spying on Trump's campaign and pre-presidential activities.

Ryan made no secret of his plan to leave the House, as  Rick Moran  blogged a couple of days ago.   The key now is who will replace him as speaker, or minority leader:

By any measurement, Ryan's tenure as speaker has been a failure.    He has failed to keep the caucus together on key budget votes and let the Democrats basically have their way throughout his speakership. ...

[E]xpect a war for control between the conservative Freedom Caucus faction and the more establishment Republicans.