It depends. In the US, trademarks are intended to protect the consumer, not the company. If consumers no longer associate a name with a particular brand, then the company loses control over the trademark. So long as a company continues to defend the trademark, they maintain control of it. That is why, for example, Xerox had a massive advertising campaign for the use of the word "photocopy". Previously, "Xerox" was used to refer to making a copy of a document, and Xerox (the company) was in danger of losing the trademark.

Wikipedia has a good list of names that are in the process of becoming generic, much to the dismay of the companies that hold the trademarks.

https://en.wikipedia.org/wiki/List_of_generic_and_genericize...