For if the heavens are to be changed, assuredly that which is changed does not perish, and if the fashion of the world passes away, it is by no means an annihilation or destruction of their material substance that is shown to take place, but a kind of change of quality and transformation of appearance. Isaiah also, in declaring prophetically that there will be a new heaven and a new earth, undoubtedly suggests a similar view. For this renewal of heaven and earth, and this transmutation of the form of the present world, and this changing of the heavens will undoubtedly be prepared for those who are walking along that way which we have pointed out above, and are tending to that goal of happiness to which, it is said, even enemies themselves are to be subjected, and in which God is said to be all and in all.(Book I, Chapter 6, Part 4)

Change does not inevitably involve decay and even where there is decay this may not be the whole story.

As I age there is undeniable decay. But there is also growth.

In death there is surely an end. But there may also be transformation.

I expect what is translated above as happiness was originally a form of the Greek mακάριος (makarios). This is the blessed of the beatitudes, the virtuous fulfillment of Aristotle, the consistency with purpose of Plato.

Solon said, "Call no man happy until he is dead," meaning - I think - beyond the reach of ephemeral cares.