The single question, as I view it, in all these cases, is merely one of fact: What do the buyers understand by the word for whose use the parties are contending? If they understand by it only the kind of goods sold, then, I take it, it makes no difference whatever what efforts the plaintiff has made to get them to understand more. He has failed …. 
Judge Hand’s statement is the classic and oft-quoted definition of genericide: when a formerly fanciful term becomes associated in the public mind with a specific good rather than a specific brand, the term enters the public domain and loses its trademark status. This is a factual distinction, based solely on the public’s understanding of the term’s meaning. Consequently, there can be no benefit to the mark holder for having acted to defend its mark, nor any sort of presumption of waiver or estoppel for failure to act. All that should matter is how buyers now understand the mark, as evidenced by factual indicia like their actions, statements, or survey responses.
However, courts’ analyses of genericness are rarely this straightforward. While most courts agree with Judge Hand that a trademark owner’s defense of its mark does not prove that the mark retains protection, many of them are willing to use the trademark owner’s actions as evidence that the mark has become generic. J. Thomas McCarthy, in his treatise on trademarks and unfair competition, notes:
There is a great deal of imprecise talk in the case law as to the impact, if any, of plaintiff’s failure to sue third-party infringers. While many defendants, and some courts, talk about such a failure to sue in terms of “abandonment,” other courts state with vigor that the failure to sue others is totally irrelevant. The truth lies somewhere in between. 
The situation is even more confusing than Professor McCarthy indicates. Some courts discuss abandonment and genericide-through-inaction separately, as if they were independent categories.  Some of the courts who “state with vigor that the failure to sue others is totally irrelevant”  subsequently weigh that failure to sue as evidence in the course of the same opinion. Learned Hand himself, in the same Bayer opinion in which he holds that the entire analysis ought to turn upon the buyer’s understanding, concludes that Bayer had allowed the term ‘aspirin’ to become generic by failing to take affirmative steps to show that it was not, and that, “Having made that bed, they must be content to lie in it.”  Clearly the “single question” of buyer understanding is neither as simple nor as absolute as courts’ pronouncements would lead one to believe.
Because few courts either say what they are doing or do what they say on this count, it can be difficult to determine what is actually going on. Through analysis of a number of historic cases and a few contemporary ones, this paper will explore the ways in which genericide has been treated. It will also investigate the extent to which courts’ vagueness in defining what a trademark owner must do to protect its mark has compelled mark owners to be excessively zealous in prosecuting infringers, out of fear that inaction may be used against them in the future….