Being able to go back through two years of chats with your wife most likely was never an intended use case for Slack. For a free tier, 90 day retention seems reasonable. The issue Slack has in their pricing, for private use, is that the cheapest tier is pretty expensive. This could be remedied by adding a private tier, for $1 per month per user. I think Slack has put a large number of potential customers in a position where they are asking them to upgrade to a paid tier, but not providing a realistic tier to upgrade to.
I am fascinated by the amount of people who use Slack though. While completely off topic, it an absolutely mess of a thing. It has to be one of the worst UIs I had the misfortune to use. It makes Google Chat look reasonable.