S. R. Faruqi:

This theme Shah Hatim has presented with great eloquence [balāġhat]:

faqīroñ se sunā hai ham ne ḥātim
mazā jīne meñ mar jāne meñ dekhā

[from faqirs we have heard, Hatim,
the relish of living, they saw in dying]

Ghalib, using imaginative proofs as usual, has created a new aspect:

G{21,1}

But Mir has placed the word lażżat very well. And by saying jānoñ kā khapā jānā he has also established the implication that the point is really about lives being destroyed in passion, not about dying in the ordinary way.

Then, he has shown Khizr and the Messiah, with all their grandeur and holiness, to be less than ordinary men, because they are deprived of that pleasure that is vouchsafed to the least of humans.

After the first line with its 'informative' style, the insha'iyah style of the second line is full of power, and is not devoid of the pleasure of oppositions. It's a verse in Mir's special style.

Mir has used this theme in other places as well, but in verse that haven't managed to attain the limpidity and delicacy of the present verse. From the second divan [{664,5}]:

mustahlik us ke ʿishq ke jāneñ haiñ qadr-e marg
ʿīsâ-o-ḳhiẓr ko hai mazā kab vafāt kā

[those destroyed by passion for her know the value of death
when do Isa and Khizr have the pleasure of death?]

Also from the second divan [{996,7}]:

apne taʾīñ bhī khānā ḳhālī nahīñ lażżat se
kyā jāne havas-peshah chakkhe to mazā jāne

[to consume even/also oneself is not devoid of sweetness
how would the lust-habituated ones know? if one would taste, then he would know the relish]

One meaning of khapānā is 'to fill up' as well. In this regard, between 'empty' and khapā jānā there's also the pleasure of a zila.

FWP:

SETS
MOTIFS == LIFE/DEATH
NAMES == KHIZR; MESSIAH
TERMS == INSHA'IYAH; ZILA

It's especially piquant that Mir hasn't used general words for happiness, pleasure, etc., when he speaks of dying; instead, he's chosen lażżat and mazah , two words that evoke flavor and sensory relish, so that both are often used in describing foods (see the definitions above). It makes us reflect about dying-- could there possibly be any actual 'relish' in it? Or does the 'relish' exist only by metaphorical extension, is it really only the relief and joy of escaping from life in this world? Can that relief be so great that we should pity those otherwise-superior beings who are denied it?

The second line, framed as a negative rhetorical question, uses kab not really as a time inquiry, but to elicit the prompt response, 'never!'. For Mir assumes that we readers will of course know the Islamic story-traditional view of both of these prophetic figures.