I believe that cumulated knowledge, too.
For example, when makers tried the germanium HOT, they promptly concluded that more reliable flybacks are in order, since germanium not cope very well with excessive voltage spikes (and Ge reduces maximum voltage ratings with temperature, hence enourmous dissipator needed for this function).
Even Si are more prone to fusing with sparks than tubes. Hence, better flybacks are in order.
And we are only talking about one part; in fact, capacitors are far better in end 60's when most makers abandoned paper capacitors when polyester caps become less costly.
Old 50's and 60's resistors makes more drift than the Fast and Furious Tokyo Drift drivers
;-)
And, for sure, much more reasons I believe...