The success of a relationship isn't measured by its longevity, but by how much of its intended purpose it serves.

Knowing the end goal of what we are engaging in helps us appreciate what we've gained out of it, recognise and easily accept when it is time to let go.

Tell that to those that celebrate anniversaries in jubilees.....

You can celebrate those. It is a very good thing and it is till death do us apart for some nature of relationships like marriage but it is more worth the celebration when you know the relationship has served its intended purpose over the years.

Assuming the purpose is to have nagging wife, a few ugly kids and nagging in-laws then voilà almost every married Ugandan man should start organising for the anniversary celebrations

When has that ever been an example of purpose.
Purpose can be to make you a better person, aid you realise your dreams, grow together, Create family, accumulate wealth, learn a lesson...whatever the Lord intended it to bring to fruition.
Nagging can be a way the purpose comes to life but it isn't purpose in itself.
Also in most cases what people call nagging is them failing to play their roles and being called out for it.