I'm not sure they can play .500 baseball and still be in the WC hunt until the very end. Lets say "in it until the end" means within 3 games of a playoff spot at the end of the season. Over the 2 years the current postseason format has been around, the average record of a team that finishes 3 games out of a WC spot is 84-78. While the Dbacks and Marlins made it with 84 wins last year, every other WC team has had at least 87 wins. To hit 84-78 and "stay in the race", they'd need to go 52-43 the rest of the season. To actually hit the mark that the average WC3 team is at, they'll need to go 55-40.
Now I know the NL only had 4 teams that are more than 1 game above .500 this year, so its possible the 3rd WC team is actually around 82-84 wins. But there are 8 different teams that are between 1 game over .500 and 4 games under .500. Basically a bunch of teams are bunched together playing for 2 playoff spots right now (WC2 and WC3). I think its pretty safe to assume a couple will drop off the map (maybe Washington and Pittsburgh), but I would also assume a couple of them get hot as well. The Cubs might be one of those teams, but if they aren't, playing .500 ball for the rest of the year is likely going to have them out of the race by mid-September.