I would argue that restoring a Leica III or Contax II to good working condition is preserving the best that 1930s mechanical and optical technology had to offer. The same is true of premium photography equipment of most generations said:
I believe strongly in preservation and restoration. It is vital to preserve our history. Cosmetic-only restoration is a crime, as it often leaves a pretty, non--functional, hollow shell.😱
Most 50+ year old Leicas and Contaxes are not going to be in good working order unrestored. You can do a mechanical-parts-only restoration (a.k.a. CLA); on a car this would result in a 'driver', a beat-up vintage car that runs well. Or you can go full tilt, do 'body-off-frame' resto, make it look and function like it did on 'Day One'.
Occasionally you come across 'survivors', unrestored, mint originals. These ARE priceless and SHOULD be in museums.
Look at this from the perspective of the 'old car hobby'. If a 1954 Cadillac Eldorado Biaritz is found in some old garage, it will likely need restoration to be driveable again. A 'concours d'elegance' quality resto would greatly improve the value of the car. An amatuer (shade-tree rattle-can) resto, while perhaps making the car driveable, would look spotty and not be worth much, except perhaps to the owner.
Restos have 3 levels: Mechanical only, to make it work; Amateur, to make it work and look a little better; and professional, to make it look and operate like new. Considering the age of most of our RF cameras, one of these three will be needed. A true professional resto neither raises nor lowers values; it makes it equal to the value of a mint original, which is what the result of a pro resto should be. If the object is to CHANGE the color,or options, so it is no longer 'box stock', that is a personal choice. Usually, unless flawlessly done with period-correct parts, any customizing will lower the value of an antique or classic.
My belief, be it old cameras or old cars, is 'keep it stock, stock rocks'.
Of course, that's just me. I can still enjoy a good hot rod or lead sled when they're done right. It's personal taste. There's enough M3's and Contax II's around. Cover yours in fake leopard skin, titanium plate, and dayglo Hemi Orange for all I care.😛