That denotes the cups being highly valuable or one-of-a-kind, though the sentence retains its objective, I am not sure it's quite correct
I do not understand the ending of 'diese'. It is plural and nominative (correct?), so shouldn't it be 'diesen'??
it means "one" in the sense of an arbitrary person as the subject of a sentence as in "One cannot replace these cups". In English it is very often replaced by "you".
listening to a computer pronounce this is certainly not clear that this is intended to be plural Tassen. There is not any grammatical indicator elsewhere in the sentence suggesting plural. Both diese Tasse and diese Tassen should be correct.