First, Buddha didn’t say there was anything wrong with ordinary worldly happiness as such. Except that it is conditional, as you say.

This is what Walpola Rahula says about worldly happiness, basing his answer on the Pali Canon:

"The Buddha does not deny happiness in life when he says there is suffering. On the contrary he admits different forms of happiness, both material and spiritual, for laymen as well as for monks. In the Anguttara-nikaya, one of the five original Collections in Pali containing the Buddha's discourses, there is a list of happinesses (sukhdni), such as the happiness of family life and the happiness of the life of a recluse, the happiness of sense pleasures and the happiness of renunciation, the happiness of attachment and the happiness of detachment, physical happiness and mental happiness etc.

“But all these are included in dukkha. Even the very pure spiritual states of dhyana (recueillement or trance) attained by the practice of higher meditation, free from even a shadow of suffering in the accepted sense of the word, states which may be described as unmixed happiness, as well as the state of dhjana which is free from sensations both pleasant (sukha) and unpleasant' (dukkha) and is only pure equanimity and awareness—even these very high spiritual states are included in dukkha. In one of the suttas of the Majjhima-nikdya, (again one of the five original Collections), after praising the spiritual happiness of these dhyanas, the Buddha says that they are 'impermanent, dukkha, and subject to change' (anicca dukkha viparinamadbamma). Notice that the word dukkha is explicitly used. It is dukkha, not because there is 'suffering' in the ordinary sense of the word, but because 'whatever is impermanent is dukkha' (yad aniccam tam dukkham). "

So worldly happiness is fine. But it’s still, as you say, conditioned and subject to change. So, even when it is “free from even shadow of suffering in the accepted sense of the word”, it is stiil dukkha. Even if it is just pure equanimity and awareness, still, if it’s a mental state, or trance or condition, it’s subject to change and that makes it dukkha, or unsatisfactory. Not in the ordinary sense of suffering, as there is no such in that state. But just that it is impermanent and subject to change.

So realizing Nirvana is seeing the truth. It’s not the result of anything, and is not a mental state, but just is.

Walpola Rahula put it like this:

"It is incorrect to think that Nirvana is the natural result of the extinction of craving. Nirvana is not the result of anything. If it would be a result, then it would be an effect produced by a cause. It would be sankhata ‘produced’ and ‘conditioned’. Nirvana is neither cause nor effect. It is beyond cause and effect. Truth is not a result nor an effect. It is not produced like a mystic, spiritual, mental state, such as dhyana or samadhi. TRUTH IS. NIRVANA IS. The only thing you can do is to see it, to realize it. There is a path leading to the realization of Nirvana. But Nirvana is not the result of this path.You may get to the mountain along a path, but the mountain is not the result, not an effect of the path. You may see a light, but the light is not the result of your eyesight.

…

In almost all religions the summum bonum can be attained only after death. But Nirvana can be realized in this very life; it is not necessary to wait till you die to ‘attain’ it."