"Back in the day" engineering things to last a precise amount of time was expensive. You couldn't just make your simulation software figure it out, you had to test, consult data, etc, etc which was often not done (or done less) and designs relied more heavily on data from past examples, tribal knowledge and educated guesses. Several expected lifetimes of the object later the only stuff left in service is the good stuff.

That could be part of it. But I think survivor bias generally gets much more credit as an explanation than it deserves, and I strongly doubt that older generations of tools would have been designed like modern tools even if, say, simulation software had been available then.

It could be just relentless pressure to reduce costs, after the people who knew why it should be done a certain way had all retired, and a customer base that changed into one that either didn't know how to recognize quality, or valued it less.

Maybe we can do without heat-treating the surface? Maybe just give a quick grind instead of hand scraping the ways? Maybe outsource the castings instead of doing them in-house using the process we'd honed over decades?

If you dive into the history of Stanley's woodworking hand planes, you'll see a 140 year history of bamboozlement going on. A sort of tug-of-war between consumers valuing quality and Stanley cheapening out on designs. You have their subpar Definance line, their Handyman line, and probably others from them. They possibly sold under other names as well. Their main line also went under various changes through the years. They've been putting out junk hand planes since the 1940s and perhaps earlier.

A modern Lie-Nielsen is easily as good as the best Stanley planes.

Old tools are hit or miss. Sure, the design on a drill press hasn't changed in 50 years. But the newer model probably has less runout and a better motor. Bandsaws are the same. One old tool you could never pay me enough to use is an old table saw, or radial arm saw. I like my limbs, and the only saw worth paying money for right now is a SawStop. It's like asking someone if they want to drive a car without seat belts or airbags.

The Kitchenaid mixer is practically a trope in the "buy it for life" subreddit - how the old ones were built to last and the new ones are pieces of shit because they have plastic gears that'll break if you even look at it wrong. Well,

1) those plastic gears are sacrificial gears that will break if you drive it too hard but they cost nothing and are easy to replace; the "properly built one" may have a burnt out motor at this point and may be too costly or impractical to repair - and

2) you don't see the lot of them that haven't survived to this day and whose owners have thrown them out and forgotten about them already. And

3) people keep comparing the lowest end appliance from the 50s with the lowest end appliance today, and obviously the modern one is going to be a piece of disposable garbage. The disposable garbage tier didn't exist back then so compare the quality tier then with the quality tier now, and you'll find that modern quality stuff stands up just fine, if not better because of material advances and minor innovations. Yes, sadly, there are probably categories where the quality tier has gone away and there's only garbage left, that is unfortunate.