This rule is not so strict, as in my specification, we never pronounce 甲苯（methylbenzene）,乙苯(ethylbenzene), 苯甲酸(acetic acid) in that fashion, though no ambiguity is produced, it is just weird and funny to pronounce so.
However, for familiar words like 奶奶，姐姐, the other extreme is present, which is they are always pronounced as 21-5. (In my opinion, this is an abbreviation of 3rd tones (214) rather than sandhi, as sandhi actually refers to the change of nasal sound between [n] & [m] according to the consonant after it.) This may be due to the level of serenity of words.
For 孔子 and 老子, who are generally regarded 圣贤 and widely respected, it is just improper to do so. (By the way, in the case of 老子, pronouncing it in 21-5 literally means "your father", which is "I" in a very rude way, contrary to 小子, also pronounced 21-5) The only people I have ever know was my professor of classical Chinese, maybe to add some humor to the classroom or just because he himself is "familiar" with them， being old friends in books for him.