Steem could be adapted to store the data in any manner (a database for instance). Many services keep data forever (facebook, google, youtube, etc.). The problem is that if Steem really picks up i popularity to anything approaching those levels, it will be impractical, if not impossible for the average person to operate a node. Even if there were some sort of pruning. I guess another possibility may be to drastically increase the number of witnesses but each witness would only store part of the blockchain. Any of these things would require drastic changes though.