Do all data really have to be stored forever or could some tag for when, as in blocks from when created, it's ok to remove the data be set?
Each node that stores the blockchain can decide when to prune old data, but there will be 'archival' nodes, that store all the data, all the time.
Which incentive have archival nodes to store all the data?
Hard drive space is cheap, you need only very little incentive to just keep the data laying around. Doing any sort of statistics may be fun enough for many.
To provide a better service to its users. Or just to spy on them.
;) The Blockchain database is the new Web-Archive. Do not use the Web if you do not want to be digitally remembered.
