Haspelmath M. Why is grammaticalization irreversible?

In: Linguistics 37.6, p. 1043-1068.Grammaticalization, the change by which lexical categories become functional categories, is overwhelmingly irreversible. Language change is shown to result from the cumulation of countless individual actions of speakers, which are not intended to change language, but whose side effect is change in a particular direction. Grammaticalization is a side effect of the maxim of extravagance,
that is, speakers’ use of unusually explicit formulations in order to attract attention.Irreversibility
Confronting irreversibility
Previous attempts at explaining irreversibility
Explaining irreversibility