1) It's not necessary for Democrats to win this seat for them to do well in 2018. Republicans won only one House special election in 2010 and that was the quirky jungle primary in HI-1 where Democrats actually got 58% of the vote. They won no special elections from the Democrats in 2014. Yes, they had the big senate win in Massachusetts, but they actually didn't win the senate that year. Winning this seat won't give them the majority. Winning 24 seats in November 2018 will. And they don't need a win to be enthusiastic.
2) Last week some people jumped on the KS-4 result, saying that Republicans won a district Trump won by 27 points by only 8. That's a gain of 19 points! If they can do that everywhere they'll gain over 100 seats! Trump won GA-6 by 1.5 points. Republicans won it by 2 in the special. Oops. So now they're pointing out that Tom Price won the district by 23. That's a gain of 21 points!
3) There are a few problems with applying that everywhere. Jon Osoff spent $8 million, a total Democrats won't match in swing districts in 2018. This was the only race and got 100% Democratic focus. Republicans couldn't push one candidate, only run against Osoff. The biggest difference, however, is that there might be few open seats in competitive districts. Running against an incumbent can be tough, as he or she has a ton of capital with the voters. Open seats are much easier to win and Democrats likely will be favorites in any open swing district. So far six Republicans are retiring. None are in swing districts. At least one Democratic seat, MN-1, is one that should be competitive.
4) Republicans are looking for reasons a wave isn't building, a few of them are detailed above. That's denial. A wave could be building. If one is, these are the results you expect to see in special elections. One thing Republicans can't change is that their party has control of the White House. As they know, that can be a big motivator for the opposition. I doubt opposition to Donald Trump is going to drop. The party with the White House is less motivated. Many people will feel the President they voted for hasn't lived up to expectations. People just expect too much and are disappointed. So they don't go to the polls in mid-terms.
It's way too early to judge whether there's a wave building. In 2010 there were real indications until January of that year. In 2014 there were none. The wave was a surprise on election night. But a wave could be building.