Except for tubes, today's parts (discrete transistors, ICs, capacitors, resistors, inductors, diodes, etc) are simply superior to even the very best available 50 years ago.
Not necessarily true. I have components using teflon insulation and exotic materials dating back to the early 50's. I have components from the 50's with metallurgical properties that are now too expensive to produce.
The magic is in the circuits, their design and topology etc. You can put the best components made today into a circuit but if the circuit sucks, you'll just expose the flaws - lipstick on a pig.
A good example would be my old Theta tube preamp from the 80's - its unmodified apart from some coupling cap upgrades, still the original power supply components, resistors etc - but it's good enough to pull out of the cupboard, and see off many much vaunted modern high end phono's that cost north of $5k, particularly in the area of information retrieval, resolution. It decimated my Klyne System 7 phono. Last time I took it over to a guy that was auditioning an Audio Reference top of the line pre in his home - the Theta smoked it.
The most obvious improvement in today's high end components should be stability and low noise floor. I say should be because it often isn't.
But how do you explain a product like the CH where you can buy a phono for $70k and then buy another for another one for $70k to run mono, which according to users sounds a lot better, then spend another $70k for an additional external power supply, which according to users sounds a lot better, and then another $70k for another additional power supply to get to dual mono stage and power supply ( 4 boxes ) - $280k all up. It's a joke.
High end is now about bling and brinksmanship. They get away with it because folk no longer know what real music actually sounds like. Go to any audio show, the higher the fi the less it sounds like you hear at the local concert hall. And unfortunately when you compare some the top components to some products of the past that were exceptional, from massively expensive solid state that is not that transparent to exotic valve gear that presents a horribly coloured sound with speakers that are horribly coloured to match.
Years ago I auditioned the $1m full monty Kondo system - sounded very nice until I I put a full orchestra recording on - no power, no control over musical timing - a mess. It fell apart.
Similarly I have heard a few ( I won't name them ) current much vaunted as state of the art, $50-100k solid state components that are not as transparent as my trusty modded Marantz 7/Bel 1001 monos. Dogs, expensive dogs, unless you are buying them for the price, number of boxes, looks and want to impress the neighbours with your moolah.
The good news is that there are good products out there, but you have to look hard to find them.