I have just finished chapter's 5 and 6 of The Heart of the Buddha's Teaching where Thich Nhat Hahn explains why Duhkha should be replaced with Nirvana.
For me, I agree with TNH, for if Nirvana is attainable then Suffering has ended but existence has not so then suffering cannot be a true mark of existence. But Nirvana is always there to be found once we clear the obstacles out of the way that prevent us from experiencing it.
If Suffering were a constant to existence then all hope is lost and the path to liberation has no destination and cannot be attained.
What do the rest of you think?