I don't think we'll see a final node in our lifetimes. Improvements are slowing down and will become a trickle, but that doesn't mean research stops entirely.

Consider other mature technology, like the internal combustion engine. ICEs have been improved continuously, though the changes have become marginal as the technology matured. However, if research and improvements on ICEs ends entirely it's not because the technology has been fully explored but because they're obsoleted by electric cars.

I thought the drivers of cost are lots of design work, patents, trade secrets etc. involved with each process. If there’s a “final” node, those costs should decrease over time and eventually become more of a commodity.

That's only true if the supply satisfies demand.