I do not completely agree with you on this specific point, for privacy reason you may not want any recovery key to be able to decrypt alone, if the lowest recovery condition is a threshold forinstance.
Let say my Liana policy is or(A, and(thresh(2,B,C,D), older(timelock))) where i’m A and B,C,D are my heirs, I may want to limit the possibility to decrypt the backup only if 2 of my heirs cooperate (or even 2 heirs + a lawyer/ third party).
In this (particular) case SSS (or any mechanism enforcing a threshold at decrypting) may be useful.
But I personally see it more as an optionnal feature or a different format/version rather than default.