Completely agree, when I was deciding what to go for I kept coming back to an e89 because nothing else had an interior that felt as special, in my very humble opinion of course.
I was very tempted by a V8 Mustang... interior is as cheap as my Focus.
Mercedes SL/SLK... Good quality interior, but more than a bit dull.
Jaguar F-Type... interior looked good when it came out 4(?) Years after the e89, but has not aged well.
Porsche Boxster... Nice-ish interior on the 781(?) but like the SL, not very interesting.
And they're all going to cost 50% more than the Z4, minimum.
Not meaning to slag off other cars, I feel a little envy about each of those cars for different reasons when I see one go past, but for me the interior in the e89 is just a better design.