My Unicomp Classic case has more flex to it than my Model M from 1992. Although I haven't weighed both, my Unicomp Classic also feels lighter(and I'd bet money this is objectively true).
You don't need to bet money: it's a known fact. Unicomps have a thinner backplate, so they're lighter.
All Model Ms clip together at the front, with screws at the back. All my Model Ms have a little flex when you squeeze them at the front, while at the back they're solid. My oldest Model Ms ('86 silver labels) are noticeably stiffer, probably because the ABS plastic is old.
I don't consider either of these factors to be positives. They don't improve the typing feel, and the combination of the extra weight and brittleness make the board more fragile. I once accidentally smashed an old Model M by dropping it off a table less than 1 metre high onto a wooden floor (oh the shame).
You also have to take it into account with shipping: with old Model Ms you're paying more for the weight and there's a chance they'll pop rivets because of the brittle plastic. Unicomps don't have that problem.
About the only thing I really prefer about my Unicomp board is that it emits less ping.
I think that's because of the thinner backplate.
Overall, in consideration of weight, flex, and solidity, my Model M's simply feel sturdier. As such, I surmise the original Model M's will last longer.
Your subjective impressions won't change the fact that Model Ms don't live forever (tragic but true).
Apart from the stiffening of the plastic, it's a fact that metal fatigue causes helical springs to soften with age. My old Model Ms feel a little bit more soft and mushy than my Unicomps, and some of the most heavily used keys (like shift) are worse than others.
That isn't to say I don't think the Unicomp boards are a decent clone, but to me that's exactly what they are; a decent clone rather than a replica.
You might be aware that Unicomp is the same people using the same equipment: they came out of Lexmark. "Clone", "replica", whatever -- they're modern models of a product that's been manufactured for decades, and which has changed over the years, just like with cars and any other manufactured good. You like the old ones, I like the new ones, so we're both happy.
