1. Reddit has banned and deleted countless users, comments, and subreddits over the years. All of that data is gone forever. The decisions were arbitrary and opaque. The subreddit owners and users weren't even given notice so they could back up the subreddit data and migrate elsewhere.

2. Reddit has and will increasingly demand users access the remaining data using increasingly user hostile channels. The mobile site Reddit experience is all but ruined. They're currently trialing blocking it entirely. The standard desktop website is atrocious. They track everything, and aggressively monetise, meaning ads everywhere. If the data on the site is gated behind intrusive tracking practises, a hostile UX, a plethora of ads, and perhaps in the future even a subscription, do we really have access to that data?

3. Reddit has an incentive to promote advertiser friendly content, and hide other content. This means the main feed is full of content I don't care about. Their search is one of the worst in the industry, and it has been blocking undesirable content for years. The most effective way to find content on Reddit is Google, but Reddit is working hard to break that. About a year or so ago Google could no longer accurately record the submission date, meaning their date searching tools were rendered useless. Shortly after that, Google stopped caching Reddit content. We should expect that Reddit has been working hard to ensure no one can find the "bad" content, and only ever lands on the "good" content. If you can't ever find the relevant data, do we really have access to it?

I just don't see how you could argue that our data is safer with an evil corporation than an army of dedicated users. You claim that mods and/or users are vandalising the site on the way out. We're really just deleting the content we uploaded.