Old mechanicals have a proven track record for being rock-solid and outliving the PCs they came with. The buckling spring mechanical switch on the old IBM Model M keyboard is really durable, and loud. Add in the metal backing that was tough enough to almost chop wood and you have a very survivable piece of hardware that can still be used on modern systems thanks to a few adapters and backwards-compatibility.
Modern mechanical switch keyboards don't have the years of real-world use to prove their reliability, but Cherry MX switches are given insanely hard testing that goes beyond the standard environment, so they can say with confidence that their product will work no matter how hot, cold, humid, or dry the environment is, and that you can hammer on it like the Hulk on speed for days without error or losing the 'feel' of the keys.
Old or new, with a modicum of care and selecting a quality brand, you can have a nice, solid keyboard that will remain useful long after the PC you bought it for is obsolete.