Again you and I differ in our understanding - I think 征引的主要书目 does not mean 'definitions from historical dictionaries', rather it means 'the principal classical texts cited'. So, 春秋, 左传 etc - what this is saying is that the previous work relied for the most part on about 80 classical texts, while this work has expanded it to over 200 (ie, this is the project of tracing the sources and examples of usage back more accurately and completely that I mentioned). Again, this is the same process that HYDCD relied upon - 征引书目 is what these classical dictionaries do. It is also very much a case of 'the more the merrier' - can't imagine why anyone, specialist or not, wouldn't prefer a greater range of texts relied upon so as to more accurately and comprehensively trace the usage of words (again, this range relied on is logically distinct from the amount provided in each definition - the latter could certainly become potentially be overwhelming, while the former is always best expanded)
As for the example you cite of 丹 - I think you are VASTLY overestimating the difference, as the majority of HYDCD entries for this have around 2-5 classical sources cited and quoted for each meaning, while this one in the vast majority of cases breaks each usage into an individual entry (with just a few exceptions). Further, and even more significantly, notice that a majority of the entries have the ~ followed by another character, and then a definition - ie,these are compound words being defined, not additional definitions for the isolated character. Compare to HYDCD - I started counting the words with 丹 as the first character, and got bored of counting at 100 (but there were many more, seemed like potentially over 200 from a quick scan). So rather than the 10 to 1 comparison you offer, when accounting for these two factors, it is probably more fair to estimate a 2-1 or so ratio of detail on presenting single character use in favor of 故训汇纂, while HYDCD has a much, much greater level of detail in presenting compounds. 故训汇纂 claims to have 1300万字 in total, while HYDCD is listed as having 5000万字 (word count for the book, not number of entries obviously), so it is fairly clear that HYDCD is the more detailed and expansive overall.
There is one part I can agree with your interpretation - 故训汇纂 seems more suited to the 字典 role, while HYDCD is, as per the name, focused on being the greatest 词典. However, I wouldn't take this as a negative implication - again, if Pleco should decide to include another classical dictionary, then naturally it would make the most sense to get something in a complementary role with existing products, and thus a 字典 seems the natural choice. Now there are good reasons to imagine it won't make economic sense to license and process another classical dictionary (I imagine this is the case - and I'm just grateful we got as many classical dictionaries as we did!), or that it might make more sense to aim for something more basic and aimed at learners, much like the first ancient Chinese dictionary Pleco licensed (which is certainly more simple and easy to use for learners than HYDCD is - though I'd doubt the wisdom of this, as the 词典 available already serve well as basic 字典 for learners, and so it wouldn't seem to add anything). But 故训汇纂 really doesn't appear more specialist-restricted or detailed when compared to HYDCD - really, it is just perhaps just the 字典/词典 difference, and both are useful.
I'm about to head off for vacation, so I will have to bow out of the debate at this point. I hope my above points were made clearly, and I naturally respect that we might have a difference in interpretation on these issues.
I hope everyone has a great holiday season, especially Mike - who for the inclusion of HYDCD now counts as my "大恩人"