I disagree. I tried to replace my 530d with a 528i in 2016 and ended up with another 530d. It's just better.
6 cylinders make all the difference.
There's a big difference between an NA and forced induction 4cyl engine. Whilst I'd always go for a 6cyl where available after 120k miles a lot of the refinement of the diesel will be gone. Between that and a 420i there's just not going to be a huge amount of difference. Also as I said before, at that end of the scale it's not as if there aren't other vehicles to go for!