Trademark infringements are funny things...unless you are copying someone else's trademark and claiming it's yours, you don't lose the trademark to someone else...you just lose it - it's gone forever...
Escalator, Trampoline, and Aspirin were all trademarked products, but as the usage of the name become so common place and because those companies didn't manage how people used their products name, those names are now public domain...If you let your trademark get diluted, you can lose it...So if people start putting mouse ears up everywhere, and Disney doesn't police it, they will lose the right to keep that image just for them...