Excellent machinable plastics like Delrin and UHMW have been around a long time. But note that I mention "machinable". Little things like camera gears need to be not only durable, but dimensionally stable. Even ordinary brass is a miserable material. The brass used for die-castings is a specifically different alloy; and dies are very expensive to make. Hard to say what will be 3D printable in the future. Elon Musk is 3D printing re-usable metal alloy rocket booster housings; but he has a lot more money and equipment than the rest of us combined.
I'd suspect desktop CNC is more valuable for this sort of thing than home 3D printing right now, especially as 5 axis machines come down in price (3-axis is dirt cheap, but 5 axis still costs in the 4 digit range for something suitable for machining metal). Machining delrin is extremely doable at home now, as is brass and some steel. Tool steel is an issue due to the lack of appropriate tools for reasonable money.
3D printing will be more useful for doing things like replacing a body shell or thumb grip, not precision mechanical parts. That's mostly a limitation of desktop FDM printers than 3D printing itself, printing any sort of metal starts at $10k USD and rapidly climbs from there (and you need additional equipment to turn those parts into something usable, the feedstock typically requires a 'curing' step after printing to be usable)
The real limiter though is getting CAD of the parts necessary, once you have that it's entirely possible to simply order the part from a bespoke manufacturer, either CNC or 3D printed. Costly on a per-part basis, but when doing one-off repairs is a $10-20 part all that expensive? (vs the $0.50 or less it cost the original manufacturer)
And of course, a skilled technician could simply mill the part on a lathe and/or stand mill. My grandfather (a watchmaker) did this all too often when a part for a watch or clock wasn't available and fundamentally a mechanical camera is merely a clockwork device. Such machines suitable for camera parts are readily available for reasonable cost, the limiter is skill and there remains a community of hobbyists who maintain those skills.