Right now there is no incentive for the NDP to pull the plug. None, gar nichts, rien, nada, zero. At best, the won’t lose seats, but in all probability they will lose seats. Why pull the plug now for guaranteed downside when you can wait a year for possible upside?
Now having said that, the NDP have been politically inept forever, including Layton and Mulcair. If they really wanted to become THE party of the centre left, they would attack the Liberals and ignore the Tories. By fear-mongering the Tories like they do, they push people in Eastern Canada and the metros to the Liberals, since they are seen as most likely to defeat Tories. When they do do well, it’s because the Liberals are being led by a dud that turns off even Liberal voters (Turner, Ignatieff).
They will continue to prop up Trudeau because right now there are no incentives not to. But if they were politically smart, they would go after the Liberals in next year’s election.