Speculation on what drives decay of this lexical distinction
I will offer some observations and delete when something more substantial is posted. I am a native speaker of Greek and German and an intermediate learner of Chinese.
If sticklers (correctly) remark that
Sie fahren das gleiche Auto.
means „the same model“, instead of the same physical car, opponents will say that the definitive article speaks against this and that plural exists to facilitate that meaning:
Sie fahren gleiche Autos.
In Greek, where (sadly) το αυτόν is practically dead, we will say
το ίδιο αυτοκίνητο
while actually meaning the very same. For the true meaning of ίδιος we will resort to singular or plural, both without article.
οδηγούν ίδιο αυτοκίνητο/ίδια αυτοκίνητα
This might indicate that plural and article tended to make the lexical distinction unnecessary. Although my Ancient Greek is rusty, I am very sure the distinction was pretty much alive then. I SPECULATE it might be associated with the fact that the definite article could often be omitted compared to modern Greek.
Contrary to my impression, my mandarin speaking friends assure me the distinction is observed by most people. It might have to do with the absence of the above grammatical features. You can not afford confusing
喜歡同一個人/romantically like the same person
喜歡一樣的人/like the same kind
I was fervently assured the last sentence would not be misunderstood for the one before it.
Reasons for necessity
The necessity of the distinction will occur in every culture with lawyers, even if rarely. It is therefore my belief that every modern language facilitates it, the question being just how common it is and whether it occurs lexically or by other means (Grammar, syntax).
In the US you can be convicted twice for the same kind of crime (für das gleiche Verbrechen/για ίδιο αδίκημα) but not twice for the very deed (für das selbe Verbrechen/για το ίδιο αδίκημα).
My roommate and I may use the same type of toothbrush (gleiche Zahnbürsten/ίδιες οδοντόβουρτσες/一樣的牙刷）, but hopefully not the same one （Die selbe Zahnbürste/την ίδια οδοντόβουρτσα/同一個牙刷）!
In the course of a criminal investigation involving shooting there will always be interest in whether two killings occurred with the same type of bullet (mit den gleichen Kugeln/με ίδιες σφαίρες), or with the very same bullets (mit den selben Kugeln/με τις ίδιες σφαίρες) .
My brother and I pay the same insurance fee. We are entitled to the same kind of beds in hospital, but not in the very same bed to be shared!