It’s fairly easy you calculate the MTTF when you design anything. That said it’s probably not as malicious as people might think their goal isn’t to artificially lower the MTTF to just beyond the warranty period but rather that they don’t put any effort in extending the MTTF beyond that.

And really it’s not that companies in the past did that much better, it’s just that in the past getting replacement parts was easier and you had fewer integrated components. Today everything is pretty much in a single package even basic electric circuits like a bridge rectifier. So things got smaller and are more likely to fail and they are harder to replace.