This has been discussed many times here and in particular on
http://www.straightbourbon.com's forum, but not lately.
Entry proof is probably a factor although I understand the Maker's Mark entry proof is notably low, somewhere around Wild Turkey's, and it is (today) hardly distinguished in the view of many. For that matter, I find Wild Turkey fairly ordinary too these last years, excepting Rare Breed - whose ichor derives from a kind of vatting IMO - yay for vatting - and one or two of the highest-end WT products. But nonetheless one would think that lower-proof entry does favour good whiskey since the old stuff was entered for generations at under 115 proof (the limit is now 125) and a lot of good whiskey was made in that time.
So first, I would say, yes, whiskey seemed more full-flavoured in the old days, richer and better than today. I have tasted many oldsters, some I bought myself off the retail shelf in the last 10 years, e.g., 60's and 70's Beam decanters, lots of National Distillers brands too, but most of that is sold off. Still, those who attend SB gazebos can taste many older whiskeys which the members bring to the gatherings. So I've had my share, some of it was a little tired - oxidized or otherwise off - but a lot was great.
But there are many other variables to entry proof, so many that it is hard to say why whiskey is blander today in the estimation of some.
The reasons IMO are:
1) Aforesaid entry proof change although this may be the least important
2) Fewer producing distilleries than 30 and 60 years ago
3) Newer wood being used to make barrels, i.e., the tree stock isn't as old as when virgin forests were still being tapped for lumber
4) Yeasts are cleaner, labs do better work to ensure a consistent yeast and therefore taste anomalies and unpredicted congener content in the ferments are fewer
5) There is less wood in the fermenters and other vessels used in distilling - it is mostly stainless steel today
6) Possibly on average less copper is used in the stills than formerly.
7) Intensively raised modern grains may offer less flavour than in the old days.
These are the main reasons, taken all in all, why flavour has changed. This is not to say good and even great bourbon still isn't made. Of course it is, but the variety is not what it was IMO.
The gent that just posted those photos of the old I.W. Harpers and other vintage bourbons might consider doing a comparative tasting to bourbons of similar age today. Harper was a top brand so he'd need to choose something of equivalent standing and ditto for the others in the line-up. Some are still made e.g. Fitzgerald, and the Van Winkle bourbons are available which aspire one presumes to that character. So do a comparative, rough and limited as it is, and tell us: which is better, older or newer? Or is it a toss-up? So often (almost always that I can remember) people who have these stocks show the pics but don't offer taste notes, perhaps because most of them aren't whiskey connoisseurs I guess, not sure why. There are some exceptions and I recall John L occasionally has done this. But the more often it can be done, the more we can know whether the feeling of many that old is gold is true or perhaps just a chimera, an expression of nostalgia and longing for bygone times...
Gary