I'm not sure that anybody ever had that issue. Mine has been in constant use since it's purchase, and it's never once had overheating issues, even in 40c+ ambient temperatures. Even if it did, there's a dandy little slot in the bottom for adding a fan if you choose.
> It had some major overheating issues that produced miniscule cracks in its porcelain body. Another major flaw was that it didn't have a fan, so users had to shutdown every so often and let the vent breathe. Apple stopped selling the Cube within a year of its release.
It probably didn't affect all production runs of the Cube, but still lots of people had them, search "g4 overheating" for some examples.
There were a lot more complaints back in the day. Note that "overheating" doesn't necessarilly mean "shuting down".
Probably because you haven't experienced the overheating G4 Cubes, the liquid cooling dripping G5 Pros, the frequently crapping out on the motherboards iBook G3, the wheezing MacBook Pros, and tons of similar stuff.
There's even a saying that goes as far back as 2002: NEVER buy Apple 1gen product (like the Retina was) --or at least be cautions. Wait till the put the second generation in the market.
So nothing to do with "declining quality" as such. All new models have some ironing out of issues to do, and all models will have some problematic runs. Same thing in the PC world, be assured, just that noone buys 3 million of the exact same PC model, whereas in the Mac world they do.