Okay, lets get this straight...
The reason you think the update won't be provided to the L1520 is due to this article on WCentral:
Why you shouldn't worry if your phone is not on the official Windows 10 upgrade list | Windows Central
The reason being that the L1520 is not listed as one of the devices that will "get the update".
That page also suggests that the L1520 is too old to be worthy of an update (I disagree but that's another story). What's not clear is whether that statement about being too old is just a guess as to why the L1520 may ultimately not be eligible for an update, or if WCentral has definitive confirmation that the L1520 won't be updated. That last part is important because this is the original source for that list:
https://forums.windowscentral.com/e...m%2Fen%2Fmobile%2Fwindows10%2F&token=84ihBdaZ
and on that page MS also states the following:
While we're working hard to make Windows 10 available for other Lumia devices, the following is a first list of Lumia smartphones that will receive a free upgrade when available
Clearly, this list is not telling us which devices will or will not be updated. It's only telling us for which devices MS will prepare W10M ROMs first. More will follow! It's entirely unclear how, based on that list, anyone can conclude which other devices will or will not officially receive W10M.
I can't help but wonder if WCentral may have done a bit too much speculating here. I also wonder whether WCentral may have reported only from the U.S. perspective, where it is entirely possible that AT&T will not provide the W10M update for the L1520, whereas every other L1520 in the world will receive it (similar to how the Icon will not receive the W10M update, whereas the virtually identical L930 will).
I don't think this story is at all settled. I think you're panicking faaaaaaar to soon. If nobody can provide more evidence of the L1520 not being updated to W10M, I'm tempted to call this premature and close the thread.