Here's the thing ... people ... (talking to y'all ... ) the Photo Stream is not and never was meant for "storage" or to "keep pictures". It was meant for easily sharing pictures ... and that would be up to 1,000 pictures, and for no longer than 30 days. Those are your "numbers" to remember for Photo Stream, you are going to see no more than 1,000 at a time and also for no longer than 30 days.
Now here's the confusing thing - for people - about Photo Stream. The "actual" Photo Stream exists "in the cloud" ... while your iPhone or iPad can keep those pictures on that device (namely "just" on that device) for longer and if I remember correctly, for more than 1000 pictures ... but it's been a while since I checked that. So, it appears that it's permanent to you, because they sit on your own iOS device. The thing is ... about that ... is that while some pictures may be "on your own device" ... they are NOT in the "cloud" where Photo Stream originally resides. In the cloud, if it's more than 1000 pictures or it's longer than 30 days ... it's NO LONGER IN THE CLOUD.
As long as nothing happens with your data on your own device, those pictures stay on your device, BUT ... since it's considered temporary and since Photo Stream is "in the cloud" ... your "backups" will NOT back up those Photo Stream pictures. But, all is well ... as long as ... nothing happens to your data. However, you are skating on very thin ice, because those pictures are not backed up and you'll never be able to recover them, if they are over the 1000 count and over the 30 day time span. Again, they are "temporary" and have always been. But, many people are MISTAKENLY treating them as "permanent" and therefore get mad when they DISAPPEAR. Well, that's what happens with "temporary" ... it eventually disappears!
The idea here was to allow you to use Photo Stream for easy sharing, and then you just copy the pictures from the Photo Stream over to your Camera Roll, where they are not considered temporary, but more permanent. AND ... your Camera Roll is backed up on iCloud Backup and with iTunes Backup.
When someone says, "My Photo Stream pictures disappeared!" ... that basically means that they didn't ever understand what Photo Stream was about!