Upgrading to the K3S is a difficult decision, for sure! I'd like to have one, but what to do with my K3? I've already seen some used K3 up for sale. But is there a market for them unless priced really low? Probably not!
You can make it your backup rig. But I, like many others already have a K2 (that we still love, and use from time-to-time) in that position.
In the end, we that love our Elecraft will probably bite the bullet and upgrade. What to do, what to do? H I !!!