I would put it the other way around. Ask Macintosh of they want to add compatibility with TagSpaces :) I think TagSpaces has the potential to dominate the metadata/tagging world :D
One addition to the last comment:
Tagging tags instead of files is a workaround for the file name limit of 255 chars. Since the parents/child tags will belong to the tags, not to the files.
The issue then is syncing the tag structure across all your devices. But that's a problem that's been tackled in lots of applications. Even a simple export/import function of a settings file would be enough for that. There are other tools to sync data. And then import the settings by using adding the "tagspacesettings" folder to tagspace or define it in the settings.
If a hierarchy is added, isn't that a reproduction of a folder structure?
Or could this be translated to a new group where you tagged the tags... I hope i´m making myself clear, well here is an example:
The files tagged with "Sister", "Brother", "Mother" won't be tagged with family. Instead the tags "Sister" etc. will be tagged with family.
Then in turn the taggroup "family" about the tags "Brother" etc. can be grouped again in another taggroup "People".
That way it's just another tag group and not a hierarchical structure which in my opinion should be avoided. The way I described it above, you could create a tag cloud with related tags (with user defined relations). In a hierarchy that wouldn't be possible.
I have seen some similar software concepts which are making use of a smart tag system. The tags were just never right in my opinion.
I think this idea goes against the philosophy of the concept of tagging, which is: Everyone tags in their own way.
I don't think any system will ever be able to smart tag for everyone. I guess it might be possible to do it for one person, but as soon as that becomes 2 persons a 100% accuracy won't be achieved.
If something like this is to be implemented, it should be in a non-obtrusive way. And there should be an option to disable/hide the smart tags (and not use them in search).
I think it does now. Maybe this should be marked as completed :)