That’s not very convincing an argument, imo. PDF, for example, is certainly a “good format” (at least it contains all the images), and it’s widely available. OTOH, Word is perhaps not such a “good format”, but is still widely available. And what about (La)TeX? Is it “too good to be popular” or is it simply very good, but also not very popular because it has a steep learning curve?
Interestingly, most of the apps supporting TextBundle are macOS/iOS only. There’s only very little support for Windows and Linux. Perhaps because developers stopped reading the specification after the paragraph stating that TextBundle uses Apple’s Package as its container format – the next paragraph explains that ZIP is also possible.
IMO, TextBundle offers little to no advantages over a DT group with a subgroup assets storing all the images etc. You can save that group as a ZIP archive and have basically a TextBundle. Minus the JSON file, but that has nothing to do with the assets. But I guess a simple script could provide for that, too. So, what’s the point in supporting a format explicitly that is already supported implicitly?