You will still be able to access your current software... so if the issue ever came up you could open with your current PS edition and re-save the file. I don't think there is any real reason to worry..
There are no guarantees of that either. If you are using Windows the past is also littered with software that will no longer run on the current Windows platform. How long will it be before the OS has changed enough that CS5 will no longer run? One year? Five years? Who is to say.
For that matter how long will it be until the media you have stored them on is no longer readable? CD and DVD formats are subject to change over time. Tape formats are constantly changing. Hard drive interfaces make changes over time as well. I have hundreds of old 3-1/2" and 5-1/4" diskettes in my closet with nothing at all to read them with. I have a 3-1/2" diskette drive, however my current computer doesn't even have an IDE connection on the motherboard, all of them are SATA-3 now.
At work I have dozens of old projects archived on DVD. At some point in the future, should we ever have to revive one of them, it's entirely possible we'll have to find an old computer with an antique DVD drive to be able to read them. When I left a company about 20 years ago I left them with hundreds of archived projects on 8mm tape cartridges and 9-track tapes. Both of those formats tended to die a few years later so I have no clue what they did. If they were smart someone moved them to a format they could use but then they were never accused of forward thinking either.
The bottom line is that for many people it truly IS, or may become, a valid problem with no realistic answer. We can't predict the future now any more than we could 10 or 20 years ago.