For the record, ALL devices days are numbered. Software and hardware "evolve" (won't say improve because this is not always the case) and s software developed to utilize new tech, it out grows older tech.
As far as the OP comment, I am left in the dark as to why you feel the way you do. The 520 was introduced as a budget device so it was already on the low end of specs. MS determined after testing that the 520 and other devices with 512MB of RAM wouldn't allow for a good W10M experience. It happens that way. I moved to a 640 and love it with W10M.
For it's time, the 520 was an excellent choice for a budget phone, but it it's time to move forward. The caveat with budget phones is that typically at best, you can pull two solid years out of them at release (especially on android) before having to move to another device to get the latest and greatest.