Do the pundits have their heads up their asses? Ticket splitting was at an all-time low this year, and Democrats are defending 5 seats that Trump won by >20%. Donnelly, McCaskill, and Heitkamp can start looking for lobbying jobs now (or not if Trump is serious about ethics reform). Montana and West Virginia actually split tickets for populist Democrats, so Tester and Manchin could possibly hang on. The same factor might help Brown in Ohio, but he also has a steep hill to climb. Nelson, Baldwin, Casey, Stabenow, King all represent states fully or partially carried by Trump, but will have a slight incumbent advantage. Even Kaine, Klobuchar, and Heinrich are not out of the woods yet. I also forgot that Menendez has his own issues to deal with.
The only real pickup opportunies are Heller and Flake, who both to Democrats' credit ran behind Romney, and might be at risk for retaliation for their disloyalty.
So basically, Republicans will almost certainly gain at least one net seat assuming Trump's approval rating is above Dubya's. They probably pick up between five and seven if Trump phones in a mediocre performance. Republicans could possibly have as many as thirteen seats if Trump completely shocks the pundits in his first two years, but it's not like he's ever done that before, right?
Ticket splitting actually was fairly heavy in MO and IN, just Trump won by way to much.
And you are assuming every single election is going to be the same. We have no idea what is going to happen the next two years.. None..
Just 2 weeks ago everyone assumed Clinton would win easily and 2018 would be a Democrat blood bath.
2018 could end up like a 2002 midterm. We have no clue at this time. But even in the 2002 midterm, gop gains were not that great and Democrats won governorships in some Republican territory.
I realize that I have a very controversial opinion here, but before next summer it is hard to get a feel for anything.