At a conceptual level, businesses aren't a static thing that are owned by a single corporation forever. Some businesses start off as family owned small businesses, or grow in exchange for outside investment, etc. When a parent passes on a family business to their child, something about that business has changed, of course. But should the child, or the grandchild, or future generations lose out on the name after 50 years?

Moving up the scale in size/complexity, you'll have partnerships between multiple owners/partners, who might want to work together temporarily, or for as long as they can, on some kind of joint partnership. A band, for example, wants to release songs under their band name, even as members might change over time.

And for larger corporations, there's a value in licensing, as well. The local McDonald's might literally be a family owned business, but franchised under strict rules by which it may use McDonald's trademarks in exchange for adherence to corporate's prescribed procedures and policies (and lots of money).

Some kind of expiration date doesn't really make sense in that situation, either.

Either way, brands and reputation last more than 50 years, and do actually provide value to the consumer. Even from the largest corporations in the world.