Today's digital cameras getting into their 40? None, not even a single one will make that long. The reason being all the crucial electronic components will simply get old, and there will be no replacement parts to salvage in 10-15 years. And it's not "old camera", when it's crucial 30% was replaced. Does anyone make replacements for late 90-s CCD sensors? Not a single model is being produced more, than a couple of years without revision. In 40 years, however, I expect the gear to have adaptable qualities. You want T50, it becomes like T50, you want D3000, it becomes like D3000, just add water... It kinda starts now, with 3D printing and flexible electronics.

Modern, computerized society lives on a very thin and temporal hologram. This occasionally breaks, like when to run a drum scanner, you need to set up a VM with Win 95 on it and purchase SCSI interface card. Wait for PCI and PCIMCIA to become our past and it's all done. Same with USB, memory cards...

We live without much attachment to items and permanence, caring more about comfort, quality of service and high performance - this is what you are as a society, and this is where we're going as well. Which is all wrong and wasteful, but we kinda accept the fact, that this is how we want to live, and this is how fast the world turns. Noone curses scientists for their discoveries or engineers for making things perform better. So, I think, apart from museums, there will be no 40 years old digital cameras in working condition.

I think Zorkis are good cameras to start learning to service such stuff: crucial elements, like slow speed escapement, are "encapsulated", mechanisms are fairy simple compared to any advanced SLRs. And this is what will survive next 50 years or so. One indeed needs some strange materials and esoteric knowledge: curtain cloth, ribbons, adhesives, lubricants, solvents, and how to use it all... But isn't that the case for other cameras too? Maybe Smenas would be better to start indeed, but what to CLA in a Smena, huh? There's hardly anything in it.

Since you have one camera working, if money is not the problem, I'd rather put some lube into the rollers and pay to have the other camera thoroughly serviced in a couple of years. 5-10 years of peaceful photography is what you pay for. And this is exactly what I've done (not to justify myself, I just think it's reasonable).