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".
There is also the paragraph mentioning the L1520 as a device we should consider too old to be worth updating. It's not clear however whether that paragraph is just providing an example of why the L1520 may not be on that list, or if WCentral has definitive confirmation that it won't be updated. That last part is important because this is the source of that list:
https://forums.windowscentral.com/e...m%2Fen%2Fmobile%2Fwindows10%2F&token=38dz6aLu
and on that page MS also states the following:
Clearly, this list is not telling us which devices will and which will not receive the update. 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 device 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 this thread.