The first M3 was a racy NA four
The second was a fantastic, streetable (but not special) NA six (US version)
The third was a spectacular, if not particularly streetable (rough when cold, that type of thing), NA high revving six
The current is a racy, streetable, spectacular NA high revving eight.
I don't think any "last of a generation" arguments hold when there has not been any consistency, having said that, I think it's hard to argue this isn't the greatest of all of the M3 engines to date. Most HP, Most torque, highest revving, extremely responsive (not sure if most responsive, but it may be), and is lighter than the e46 (engine only).
Will the next be better, maybe (doubt it could be lighter with turbo and associated plumbing, and doubt it will be more responsive or as linear), but will be far superior at peak torque and HP.
At the end, only time will tell, in the meanwhile, 8400 RPM is just right for me!!!!