Even the rangefinder patch in the Nikon S3 2000 I examined was low contrast. That's not a 50 year old camera. Maybe that's why there are more than a few threads on this site that have discussed how to improve the contrast on those cameras. And old SP's are known to have a faint secondary image. The one S2 I owned was pretty low contrast too, and that was after cleaning. And this is the same issue that the original poster apparently faced.
I have thus far not have a great quantity of Nikon S body sample experience.
The only ones I have handled thus far were a handful of Nikon SP and S3 re-issues and a few really beaten S2s and SPs in local shops.
The only Nikon bodies I have owned so far are the S3 and SP reissue, both bought in like new condition (likely camera shop stock or collectors keepers before I received them and started to use them as intended).
I have used a wide selection of Leica rangefinders before that and still own a few - new and old, some odd ~50 y old ones and some just a few months old.
The Nikon SP reissue I have has about the same high contrast as the Leica MM (M240), which is the newest Leica body I have owned.
The Leica S3 reissue does have a slightly lower contrast finder than the SP or the modern digital Leicas (but it also des have a different color tint to the finder compared to the SP which might confuse my eye into thinking it has slightly lower contrast compared to the SP).
The reissue S3 finder does not have in any way lower contrast or more difficult to see rangefinder image as my just recently rebuilt M7 x85.
The modern film Leica bodies I have used (say MP, M7) have comparable finders when it comes to contrast but are in some ways different from the MM finder.
What some may perceive as the Nikon rangefinder bodies having a "lower contrast finder" surely has to do with the appearance of the sharp edged rangefinder spot finders as the Leica M have vs the fuzzy / floating Nikon rangefinder patch leading one to perceive the Nikon finder as "lower contrast" where it is really not (at least when comparing apples to apples - cameras made approximately around the same time and in similar condition).
As mentioned earlier - age does influence finders negatively too of corse - the finder of my 1.1Mio M3 everyone praises so much is not really that much of a revelation (lower contrast than both the S3 and SP Nikons I have used) but that surely has to do with the fact it never has been cleaned so far. I have seen Kanto rebuilt M3 finders which were spectacular and at least as clear and contrasty as latest Leica digitals.
Another point I have found with the S3 and SP finders is that the actual alignment of the eye to the center of the finder is rather critical - certainly more critical than on the Leica M bodies. If positioned properly though finder contrast and focussing are no issues at all and they compare nicely to the most modern of Leica M finders in actual use.
I can imagine that for some people it simply is not the right finder with the "floating" rangefinder patch vs a sharply defined rectangle.
I really like the Nikon finders (and wouldn't have bought a second body if my experience with the first one - the SP - would have been in any way negative).
They are a joy to use and the more I use them and get used to the long focus throw of the Nikon S lenses, the more I like them (the lenses are without the slightest doubt wonderful, but I knew that from the beginning as I have used many of the Nikon lenses in LTM mount before).
