My question is - what if these programs are discontinued and I need to restore my files or use one of the system images I created with them? Will I be able to do so without the programs installed?
You will need the program that created the backup in order to restore it. In some cases even down to the exact version number.
In other words, are the backups these programs create all the same?
Most everyone has their own proprietary format.
Can I restore them with the native Windows program?
Will their system images be restored using Windows System Image Recovery?
again, highly unlikely.