FWP:
'The stone of your doorsill', says the lover, 'would eventually have vanished anyway, worn away by the friction of my forehead during my countless prostrations before your door; thus you didn't need to change the doorsill-stone because of the disgrace of my prostrations.' Thus the lover's extravagant, endless, undiscourageable devotion is poetically 'proved'. Not only is he not either angry or dismayed by her going so far as to change the very doorsill that his humble forehead had sullied-- he's even trying to save her trouble, helpfully volunteering the information that she needn't take so much trouble in the future.
As for the beloved, she may have acted out of discretion and concern for her public image: at least now people won't be able to notice that her doorsill has a hollow place worn into the middle of it. Or she may have acted out of sheer hostility and disdain, despising the doorsill for its pollution by the annoying lover's constant ministrations. Either way, the extremity of her rejection is poetically 'proved'.
Thus the verse enjoyably suggests a situation in which an irresistible force meets an immovable object.
This verse uses enjambment, which is not so common (verses that display it are considerably in the minority) but is also not at all uncommon. Since the grammatical subject is 'you', this is the first verse in the present ghazal to use apnaa in the standard grammatical way, with no ambiguity. The only other verse to do so is the next one, {43,7}.
On the rubbing away of stones, compare {66,2}.
Nazm:
That is, I would have performed so many prostrations that the stone would have been worn away. (41)
== Nazm page 41