Because these old forms can sound incorrect to modern ears, regularization can wear away at them until they are no longer used: brethren has now been replaced with the more regular-sounding brothers except when talking about religious orders.
Non-productive grammatical processes may be seen as operative within closed classes: they remain within the language and may include very common words, but are not added to and may be lost in time or through regularization converting them into what now seems to be a correct form.
Productivity is, as stated above and implied in the examples already discussed, a matter of degree, and there are a number of areas in which that may be shown to be true.
As the modern example of snuck from sneak shows, what has apparently been non-productive for many decades or even centuries may suddenly come to some degree of productive life, and it may do so in certain dialects or sociolects while not in others, or in certain parts of the vocabulary but not others.
Similarly a speaker or writer's use of words like raisinish or raisiny may or may not involve productive application of the noun+ish and noun+y rules, and the same is true of a hearer or reader's understanding of them.
It claims that high-frequency formations with the suffix -heid are available in the mental lexicon, whereas low-frequency words and neologisms are produced and understood by rule.