Trademarks can become genericized, which is a bit like them falling into the public domain. (https://en.wikipedia.org/wiki/Generic_trademark) However I don't know if the owner of a trademark can legally make it generic.
That’s not the same thing. Trademarks become generalised terms in the public’s mind, not legally. Two people in conversation might refer to a non-Xerox photocopier as a Xerox, but a non-Xerox photocopier company can’t just advertise their products as being Xerox.
Additionally, from the wikipedia page you linked (emphasis mine):
> A trademark thus popularized is at risk of being challenged or revoked, unless the trademark owner works sufficiently to correct and prevent such broad use.
Which, again, goes directly counter to the idea of a public domain trademark.
It becomes legal if the trademark owner can no longer defend the trademark. The difference is that here the trademark owner would somehow have to guarantee they would no longer defend the trademark. We're not worried about perifractic, but what about if he dies and someone more litigious inherits the mark? The issue if how to legally say "I will no longer defend this mark, and this will be forever."
When a trademark becomes "genericized," the trademark is lost and any other company can use it for their own products. The word "escalator" used to be a trademark and now anyone can sell escalators and market them as such.
