How much are they actually changing though? We could imagine they take literally the same design and stick a generator in the corner. Making that mandatory would not be "killing" the Lightning. At the other end is a total redesign that cuts most of the battery, which would be killing it.
They have a patent for a range extender under the bed, behind the axle. Which I guess could work, depending on how big an engine it needs to be. By my math it probably needs to make around 50hp, though for towing maybe double that. But it doesn't need to be a full size 400hp V6 or anything like that.
If they can throw it under the bed, keep the weight under control, maintain a decently big battery, and not lose the frunk ... then I'll be an optimist about this.
I'm planning to keep mine another 7 years, though, and they only announced the current lighting 4.5 years ago, so a lot can change before I'm in the market again.