The "protection" you speak of is that consumers can associate a producer with a product. In this case the producer of the original product was Sun which is now part of Oracle. Thus someone who gets an update for "Open Office" is getting it from the one true source for Open Office.

The freedoms provided by the Open-Source license allow someone to fork that code to create a "new improved product" - but that product can't be called "Open Office". In this way consumers can't think they're getting x and end up with y.

If the consumers are concerned about Open Office, then it's their choice to go to Libre Office. They are in effect opting-in to a change. Changes of this nature have to be opt-in, there's no way around that.

Allowing multiple people to use a trademark, or worse encouraging court battles over who is the "one true heir" to a trademark would not be in the consumers interest.

Clearly a fork can not have any claim on a product name because if it did there exists the possibility for infinite forks, and infinite products with the same name. This in no way serves the consumer, and specifically it does not facilitate the identification of the product, not does it serve to identify quality or other characteristics.