BTW, if you're feeling adventurous, you could even add a few more blackletter-style quirks, such as a /P with a full-height body and a descender, /H and /K with overhead arches, lowercase-style /N and /M, etc... maybe as a third export font, so as not to weigh down your default font.
It can't be that bad if the Duden as been using it on its cover...
The only reason I can think of not to use the cap ß is if it's badly designed (e.g., not instantly readable or disharmonious with the other letters), which is unfortunately rather often the case. Luckily, this is a problem that type designers can freely address by themselves.
As for «predictability»: I would argue that switching fonts will generally change the appearance of a given text more drastically than exchanging SS with ẞ... and while using SS for ẞ can cause loss of information, sticking to ẞ never does. After all, if the user typed ß before capitalization, they meant ß rather than ss. Anyone who can read ß can also read ẞ without hesitation — if not, the ẞ is badly designed.
A historical practice is not necessarily a logical practice. The practice of writing capital ä as Ä overruled Ae since it's more consistent. Similarly, ẞ should now overrule SS when capitalizing ß, since it's more consistent. We have no obligation to stick to past mistakes.