A win tonight against the Cowboys would certainly help the Jets. A loss would not be the end of their season, however. In fact, there is a plausible, even likely scenario where the Jets would control their own destiny by the end of the week, even if they lose to Dallas.
If the Steelers lose to Denver, the best record they could finish with is 10-6. The conference record they could finish the season with is 7-5. If the Jets lose to the Cowboys and then go on to to beat the Patriots and the Bills, they would also be 10-6 but own an 8-4 conference record. The Jets would get into the Playoffs because of their better conference record.
This also goes if the Steelers lose in Week 16 or Week 17. The Jets would go to the Playoffs at 10-6 as long as the 10-6 came from losing to Dallas and beating the Patriots and the Bills. If the Jets beat Dallas but then lose to either the Pats or Bills to finish 10-6, the Steelers would get in at 10-6 because both teams would be 7-5 in conference, and they would go to the next tiebreaker. Pittsburgh would win.
This game tonight does matter because of Kansas City scenarios and for opening breathing room n scenarios where the Steelers lose more than once in their last three. It also matters for the purposes of keeping the team hot heading into the final stretch. It is a game the Jets can afford to lose more than the next two, however.