I don't completely agree, but I work in storage so I'm also a bit biased.
First of all, even though the disk prices may be getting lower and lower, the amount of data being backed up is growing (almost?) exponentially. This not only requires a lot of space, it's also getting more difficult to back up in a certain amount of time (you probably want your full backups to finish before starting incrementals). There are various workarounds for these issues (compression, snapshots, deduplication etc.), but none of them have really solved them yet.
Secondly, the disks used for backups are usually part of disk arrays and those are quite expensive and also require some administration.
Thirdly, disks are only one part of the story - tapes are not dead yet and probably won't be for a while. They are usually faster than disks (when streaming speed is achieved), cheaper per TB of storage, can be stored more efficiently etc. There are drawbacks of course, so bigger companies usually create multi-tiered backup systems combining all available solutions.
As for the deletion of old data - some data has to be archived/available for a long time in order to e.g. comply with certain laws, but in general one implements multiple retention policies, depending on the importance of data. Most likely you wouldn't want to store _everything_, not just because of the space needed, but also because you need to somehow catalog what is being backed up and this also requires substantial resources.
First of all, even though the disk prices may be getting lower and lower, the amount of data being backed up is growing (almost?) exponentially. This not only requires a lot of space, it's also getting more difficult to back up in a certain amount of time (you probably want your full backups to finish before starting incrementals). There are various workarounds for these issues (compression, snapshots, deduplication etc.), but none of them have really solved them yet.
Secondly, the disks used for backups are usually part of disk arrays and those are quite expensive and also require some administration.
Thirdly, disks are only one part of the story - tapes are not dead yet and probably won't be for a while. They are usually faster than disks (when streaming speed is achieved), cheaper per TB of storage, can be stored more efficiently etc. There are drawbacks of course, so bigger companies usually create multi-tiered backup systems combining all available solutions.
As for the deletion of old data - some data has to be archived/available for a long time in order to e.g. comply with certain laws, but in general one implements multiple retention policies, depending on the importance of data. Most likely you wouldn't want to store _everything_, not just because of the space needed, but also because you need to somehow catalog what is being backed up and this also requires substantial resources.