Do you have marketing automation tools or email marketing tool that is generating a ton of valuable data? If so, when was the last time you looked at the really old historical activities or records that were created by these tools to see if they should recycled? Chances are the old data is simply not relevant. Tools such as Mailchimp or Pardot can quickly become data storage hounds and you should setup a storage recycle job today to remove this old and irrelevant data.
After a few years, it's probably safe to say that the old inactive campaigns in Salesforce are not influencing behavior and it's best to forget about the past and recycle this data. Try recycling old inactive campaigns that are far longer than your buying cycle.
NOTE: Campaign members that are directly recycled (without targeting their campaigns) are not available in the recycling bin.