Before anyone from MAXON chimes in, let me take a stab at calming your concerns.
Look at how the new material node architecture was implemented in Cinema 4D. You have nodes, you have legacy and you have something in-between called the uber-node. Not sure how you could cover the gap from old to new better than that. They even provided a very handy set of the most common shading types (car paint, etc).
Now, based on the re-iteration from the MAXON execs at the end of the presentation that they are NOT forcing anyone to use these new modeling nodes, I would imagine that they would take the same approach to rolling out Neutron (though I personally like "Neutrino" better, but let's not go there). It will be similar to how they rolled out the material nodes: You will have legacy, Neutron and possibly something in the middle.
In fact, I would imagine that when MAXON said that R20 and R21 contained updated primitives and basic modeling tools "with significant speed increases" designed to support the new core, we have already started to be exposed to that "something in the middle" without even realizing it.
Though I previously had some concerns about this, when all is said and done I am pretty convinced you will get the viewport speed increases even should you decide to stick with the legacy object manager and modeling tools. So the path to viewport improvements will not be a technically challenging one for the user....it will be challenge for MAXON though and that is why I think it will be rolled out in bits for some time.
The big question how long will that take.
Dave