>A 2x4 LEGO brick manufactured in 1958 will snap perfectly onto a brick molded this morning..

This is just manifestly NOT TRUE. The outward appearance may be the same. There were intentional improvements to the walls and tubes that make fit less than perfect. Generally, today's brick requires less force to snap and un-snap, because the compression is focused onto fewer points. (I guess this lowers the "hoop strength".)

Older bricks can be either: completely loose, or clutch so hard to each other they are the devil to take apart.

I have many bricks from 1962 onwards. The oldest 2x4s and 2x2s were made of cellulose acetate (CA) (in North America, intermediated by Samsonite.) CA were softer, and either had less clutch power to begin with, or lost it over time. When I got them in the 70s, they fit but wouldn't reliably stick to each other, nor later 70s-80s bricks (all ABS plastic by then.) (CA bricks were mostly red, and they have a pale orange tint.)

70s-80s bricks did not always age well. Aged 1x4 or 1x8 bricks can have the outer wall bowed inward slightly. This is a mold engineering problem anyway. Later, 80s bricks were improved by slightly thinner walls and some reinforcing tabs. The older, aged bricks can stick brutally to each other and to newer bricks.

The 10x10 baseplates didn't age well (these were once box-tops! Tog'l Toys also had the baseplate as a box-lid.) Possibly made of polycarbonate (PC). Other large plates in ABS-- for instance 6x16 (Auto Chassis, red) -- have warped. They were also more brittle to begin with.

So inside Brick geometry has changed over the decades. 60s-70s bricks are closer to plain boxes with tubes inside - as the Kiddicraft prototype of the 50s. In the 80s, the outer walls got thinner and had tiny studs where the studs contacted the wall. And the tubes changed from cylinders to just slightly clover-leaf inside, so that a tube over a single stud now formed 4 points of contact, and came apart with lower shear force. (I believe this also made it easier to pry a plate off of a larger plate.)

I have Fabuland sets from early 80s, whose plain bricks are so stiff, they are positively brutal to snap onto each other or 90s bricks.

The brick geometry of today is much improved. And the ABS is more "plastic", perhaps more "B" (butadiene rubber) or less "S" (styrene): I can drill it more cleanly.

Mid 80s and 90s bricks will interoperate just fine with today's. But bricks from before that period didn't age so well (and their corners, I believe, used to be harder.)