That original Triumph parts (probably all English cars at the time actually) had great quality is a fact.
When I grew up all we could hear about UK cars was their bad quality, so much so that most of them was taken off the market in the 70s, Japanese cars on the other hand was so good, and took over.
My first car was a 12 year old Toyota that I partly had to restore due to collision damage, rust and general wear and tear. The metal itself on the Toyota was thin and rusted through very easily, in addition to that a lot of screws, bolts and nuts was rusted together and often broke off when dismantling, so for me a 17 year old lad with little experience at the time I struggled to get it apart, together was easier as the Toyota parts fitted nicely.
When I some years ago bought a Triumph that was more than 40 years old I expected more of the same trouble I remember having with the Toyota, as the car was more than 3 times the age!
But not one bolt or screw was stuck, not one broke off, most things could be taken apart and repaired, not like the Japanese which was not built to be repaired but a complete new unit must be bought, I guess this comes down to the use of quality materials and well engineering.
So who got the best quality? I think the quality of the British cars was actually good at least well engineered with good quality materials, the bad assembly practices on the other hand....
The quality of the new parts is a result of the race to the bottom in regards of the demand from the public of cheaper and cheaper parts.
This problem could maybe partly be solved if the suppliers actually inform customers of the differences of a good quality expensive parts rather than the cheap and bad parts.
The parts are usually marketed with just price, with no explanation how much better it is than the cheap parts, without information most people will assume they are the same, and choose the cheap part.