I must say I am surprised with that line of thought. Xpresso is old, single threaded, slow, can't generate objects, limited in a lot of aspects, doesn't even meet the basic requirements for modern node system and as such it simply can't be updated. Scene nodes on the other hand are not a simple node system slapped onto object as an expression, they are in fact new core development where nodes are integral building blocks. As far as learning part goes, all the concepts from Xpresso easily transfer to Scene nodes. When you say "complete" I am not sure what you mean. As of right now you can literally build objects, generators, deformers, effectors, fields, rigs, setups of any kind in Scene nodes, something which Xpresso can't nor was ever designed to do so. In Scene nodes. you can import any object from rest of Cinema into it such as particles, splines, meshes. Anything that is built with nodes can be exported also. Of course, there is always resistance to new stuff, but give nodes a try, they are vastly superior, simply incomparable to Xpresso. Nodes can "spin" millions of objects which was number one request not to mention that you ca achieve things with them which previously required python or plugins. Sure, it is still labeled as a tech demo and has limitations but what it can do greatly surpasses what it can't do right now.
@hikarubr
Xpresso or Scene Nodes (in fact all node systems do) use pretty much the same principle. You have nodes, connect wires, use data types and blocks of functionality - that is it!
To someone who has experience with Xpresso, Scene nodes won't be a problem since there is no paradigm shift from user point of view. Also,I highly doubt MAXON will abandon modern, fast, highly flexible architecture anytime soon 😉