No way they will discontinue the iMac. What makes you say that. Its not their bread and butter. but Macs in general earns more and more, even if the iPhone earn apple a ton more. Also, its part of the ecosystem, without the mac, no development and no apps, it would kill the iPhone too eventually.
I suspect an early iMac update this year. The update will be incremental, only new CPU and GPU, I dont see any point for apple to wait 6 months after they are released. You can clearly see on the Buyers gui that the 200-250 days update are the small updates and what drags out the days are when apple makes a complete redesign, which is typical, then apple always gets into delays because they are doing their best to make a new awesome product.
So, I expect a new iMac this summer 2-3 months from now. imo.