You make 2 really good points here. Put a modern RF lens on a vintage looking camera and the visual narrative is immediately lost. It'll look like a modern crash helmet on a guy riding an old Harley with all the tassels and chrome. It just defeats the point. Ultimately this is a vain styling exercise.
The other point that you make is that the ONLY benefit from the older FD mount SLR's is the huge low contrast viewfinder. I still have my A-1 and AE-1P and the A-1 has the best view finder I've ever used. The EOS 5DII with the EG-s fine focus screen was good but tiny compared to the massive screen fitted to the A-1.
Over the years, the optical view finders have become more contrasty (DOF limited to F4) but a lot smaller and darker. Then we all flip to Mirrorless cameras, ditch the mirror box and prism construction for EVF's. The current best EVF from Canon in the R3 and R5 is still very poor compared to the A-1. Even at 5milion pixels, it's still lower resolution to the finer optical viewfinder from the A-1. I use the R6ii and R8. The R6ii has the better EVF resolution tot he R8 and has a better viewpoint size. The R8 has a really low resolution incomparision and it's viewpoint makes the EVF / view finder experiance the weakest part of the camera (apart from laughably undersized tiny battery).
Isn't it a bit weird that we are contemplating the visual asthetic of a camera from 1978, whihc is terrible by today's standards but ignoring it's mechanical attributes and advantages which is what actually made it a great camera in it's day. Ergo....retro is a weird anathema of specifications based on some kind of random emotion nostalgia. Can anyone discern what that perfect balance will be and will it appease anybody?
I don't agree with every point but I like what you said about the viewfinders.
I definitely don't feel that the only benefit from the film SLR's is the VF; it's just one of them. And several aspects make those alone better than DSLR's. I really do not like the f/4 limit of stock viewfinders. Also don't like doing replacement viewfinders. Also, you're right, the DSLR and EVF viewfinders are small. It's just overall not even a close experience. The film VF's are not only huge but also have a glow looking through the lens that makes images look truly special. It makes it exciting to shoot, when you look through the camera and go, "Wowwwwww"
I am probably younger than you but grew up using film SLR's. I have an EOS 650 (signed by Wim Wenders, actually), but I always found the AF film SLR's to be ugly. If I had his autograph on a Canon A-1 or F-1, or one of the older Nikons, I would have it out on my bookshelf. Instead it's buried away in some box, and I guarantee I will never use it again.
Overall my point is, you have to realize that when the 650, etc, came out, the new design style was equally as arbitrary as the "retro" 60's/70's era looks. They're just designs. Me personally, I prefer the look of the older film cameras. It's not about "older" or not. I just prefer them. When DSLR's became popular I basically rejected them -- I'm an artist and dislike things that I don't find visually appealing... I stopped carrying a camera 24/7 like I used to do with my AE-1, FM2, and F3HP. The DSLR's were not only too big but really unattractive, to me.
I shoot professionally now and will only use the modern SLR's as a professional tool; I miss having something that I actually like carrying with me.
These are just my thoughts.
The last thing I would say is that I vastly prefer the manual controls of the film SLR's. I have been shooting professionally for over ten years, and still when I look down at the top of a "modern DSLR", my brain goes kind of blank. I think that the mind is better at dealing with controls where things are separated (not on a screen or with several different adjustments available on a top LCD). And I'm not a stupid person, not even close. I think the brain functions better when one adjustment is attached to a single, mechanical level or dial. When I'm shooting, things have to be automatic; shooting professionally can be stressful and time-sensitive, and ironically, these DSLR's create a minor hiccup in my shooting workflow.
I also virtually never use the menus.
So this is just me. But I think we agree on a lot of things.
If Canon basically made this camera with a huge viewfinder, manual controls, nice design for the body and lenses, great low-light shooting, great autofocus, and 24-33mp, it would be a simple, powerful, and really enjoyable camera to use. But I think so much comes down to the viewfinder. Make it huge and beautiful, and allow me to choose to remove some of the text from the surrounding display. I also find the bright green focus box on the EVF's highly distracting. Totally hate that.