A general note on technology -- older tech of some manner or another will almost never completely go away.
Heh, hm, vacuum tubes, and horses for transportation, are probably two unusually good counterexamples. They still remain of course, but only at a tiny fraction of their peak glory, and mainly because of enthusiasts willing to bear the cost. It's fair that some stepping-stone technologies can be fully obsoleted, that's true, enthusiasts notwithstanding.

But for everything that exists as a current basis for higher technology, it's never going away, no matter how integrated that higher technology is. Single BJTs and MOSFETs are still with us, as are op-amps and comparators, CD4000 and 74(LS)/HC logic, and all manner of MCUs from the 8051 (and even 4-bit machines, for that matter) to x86. Which, heh, x86 itself, I mean...

(But not so much bit-serial machines, I think? They were great back in the days of serial memory -- rotating drum or delay line. Anyone know any current applications?)
Meanwhile, we absolutely have CPUs with stacked Flash and DRAM dies, integrated switching regulators and all manner of peripherals and controllers, to realize insanely powerful SoCs in pocket-sized packages, with short bills of materials -- in a given product, they've displaced a whole host of other components, but they haven't displaced them elsewhere, and I suspect never will.

And going further back, it's especially difficult to obsolete anything to do with handling of raw materials -- like ore mining and smelting will always be with us. The exact manner in which they are done, can vary -- compare electrowinning and refining of copper, to the blast furnace it replaced. And not that blast furnaces have gone away either, they're still king for iron as far as I know. Industrial forging is done in a manner and scale hard to imagine from antiquity, but it also strains the imagination to think of some kind of, say, 3D printing process that is able to produce a similar grain structure in as short a time. (But 3D printing also offers revolutionary opportunities in the design of intricate forms, otherwise-impossible cavities, and the use of difficult or otherwise impossible alloys and tempers, thanks to rapid fusion, directional solidification and powder metallurgy methods.)
Tim