Actually, I think it would be very easy to please pretty much everyone and they aren't that far off if they would simply show a list of calendars that you can select from and have the ability to select multiple calendars that would cover the most asked for options ... but keep each more focused. Nobody seemed to complain much about the old US Holidays calendar and it is nice to have other holidays show up like Saint Patrick's Day, Cinco de Mayo, April Fools, Tax Day, Election Day, Valentines Day, Groundhog Day, etc. The complaint seemed to be with holidays it did not have so for that have optional calendars like for various religious holidays, seasons (i.e. first day of each season), and others as the requests cross a popularity threshold. The big thing is that Apple stuck to a policy of no overlap between calendars but having several special popular special purpose one that are optional and accessible via Properties, it would resolve much of this problem. In addition, if they had an info button on each selection where you could see the holidays that will be added, then that would also help users decide what calendars they want to see. Again, if they are getting complaints about dates missing, then they could have internal guidelines based on feedback that would drive the creation of a new popular calendar or updates to existing calendar. This would probably satisfy the majority of people and the calendars would most likely become more stable over time so third party calendars would not have any overlap either. The main thing is to define each calendar and stay within the definition of that calendar for any changes.
Then continue to provide support for third party subscriptions so people could add other calendars they are interested in like for sporting events, etc.
This is all something Apple could easily do if they chose to and even include it in the next release since the structure is already mostly in place ... it's simply defining and organizing the optional calendars.