I am also sad to hear that BMW stops making the NA I6. Something is telling me that one day they'll return though. It's the best engine ever built. And, if I'm not mistaken, BMW was one of the first, if not the first, to build a straight 6-cylinder for passenger cars.
I would probably keep on driving your current car until it dies or you need major maintenance. I could imagine that it will run at least another 50k, but you might be lucky and drive another 150k. I have a weakness for older cars, so it would be cool to see that. By that time, maybe spend some serious time searching for a good second hand E90 (and keep your current BMW for the museum one day). I'm sure there will be a few low-milage cars around by then. And, if you find a well-maintained E90 with 60k on the gauge, I don't see a problem either. You might even find a Certified Pre-Owned one at the dealer...