Apple has offered me an upgrade from 5 to 5s for free?

So my iPhone has been broken for few times and it seems like the nightmare will end finally but i still don't understand a part of it…
So long story short, my phone got broken again, now the screen makes a cracking loud sound every time i touch it so i took it to apple, they repaired it - after the repair the problem didn't go away so the offered me an upgrade from my broken 5 to 5s due to many exchanges in the past, thats all nice, actually very nice from the store but i have to wait 15 days, while using the old one for those 2 weeks…
is there anyone who had the same, miracle, happened and could tell me if they really sent you the phone int hose 15 days or why 15 days? Wouldn't it be just easier for them to give me one from the stock? I want to be happy for the new phone but can't until i won't be sure it will really be given to me because it still sounds a bit weird, right? Anyways, hope there's someone actually knowing what I'm talking about because i can't find a single topic about this anywhere.

Thats great if they offered you that, its a major upgrade and 5S is worth alot more so in the future when you sell it you just got a big upgrade in value.