All ereader screens are fragile, and you have to be aware that it's not just rough handling, but any flex of the screen, or pressure on the screen that can cause the damage. Something hard leaning on the screen of your ereader while it's in your bag (like a book, your phone, the metal clasp of a purse) could be all it takes to damage that glass substrate and while it might not crack at the time, the slightest bump at a later date will cause it to crack.
Perhaps the PRS-X50 series were a little sturdier due to their metal frame (less risk of flex?) but the screen is still as fragile to any other damage. There are plenty of people who've had older Kindle & Kobo models (or indeed any brand of ereader) that still work fine today 4-5 years after they were bought. My old Kindle 3 has been dropped a couple of times but still works, and that's just sheer luck and not any 'better' build quality.
I don't keep my ereaders in a case, so they lie about the house and get used without any protection, but as soon as I take them anywhere, into a case they go.