I feel that simple rockets 2 is going to replace simple planes as soon as it launches.
It can offer all the functionality and parts simple planes has, plus a whole bunch of new rocket parts and a much, much, much larger and interactive map than the one in simple planes.
By the looks of it, you can surely build planes and vehicles (if they have rover parts) in simple rockets 2 with much higher detail and accuracy with less work (the new fuselage system looks absolutely awesome) than in simple planes.
It might not be a bad thing, but to be honest, I'd be pretty sad if simple planes gets abandoned after the new game launches.
I don't think SP will be going to far. Weapons will not be introduced at all. Andrew confirmed that. I agree with less players, but think of it as an evolution, not a abandonment. We will rebuild what we had in SP. Not just that, but SR2 will be a continuation of the stories we created. The friends will stay as well as good times. SP will not go away completely.