It has value in terms of being an example of how old these institutions are, not to mention how trustworthy and with lasting power they are.
Administrative effort + 10 bucks is _nothing_ compared to that.
Paying all and any debts makes it much easier to raise new bonds for future construction and maintenance. News articles like this are like the ultimate letter of credit.
The legitimate/legal solution would be to buy back the bond and cancel it. But I guess everyone allows to continue because it's a cool piece of history.