IMHO (and speaking without the time, aptitude or knowledge to dissect the technical/manufacturing reasons why), it seems to me that the philosophical reason why watches may not be as functionally reliable today as they once were is because they have long ceased being "functional" items, just like fountain pens have.
In days past, even with the inherent limitations of oil/waterproofing/etc, it was essential that one's watch be accurate, reliable, and (generally) capable of fitting under shirt cuffs, just as it was essential that a fountain pen's nib not skip or dry. Those were the days when we (they) used watches to tell time and pens to write, and that's why a vintage Parker 51, with its hooded nib, will outperform just about any modern FP, functionally speaking.
Today we get the time from a myriad of digital sources, and write (occasionally, when typing won't do) with disposable ballpoints. Watches and pens have mostly become jewels and symbols of status - hence the innumerable "limited editions", exotic materials and hefty sizes that are the antithesis of functional convenience, at least for pens. Not that these phenomena did not exist in the past - they just did not represent as large a proportion of the market...
I tend to believe that the old days of non-disposable, functional items were "purer", but then again this is just my personal feeling and implies no criticism of the modern attitudes. Things are what they are. Like AaronM says, there may be a "selection bias" (in an outcome research sense) that skews our perception of old calibers towards the positive, but I am still grateful for the vast availability of "survivors" from the 40s-50s, housed in 36-38 mm diameter, 8 mm thick cases I might add...
Best