Particularly in the context of the current API decisions, the thing Reddit is doing is closing off access to that data. A user saying, "fine, but if you're not going to allow open access then I'll stop treating my content like it's open access and I'll remove it" is I think a pretty reasonable response (both for the user and for society). The alternative is Reddit gets to lock down that data, but because it hasn't gone all the way yet and completely locked it down, everyone is still obligated to let them keep that data? It just doesn't make sense.
It was brought up above, but Reddit archives exist. So it's not necessarily like the content is entirely lost. But it's less convenient to access, and that's purposeful, because it's bad for society if we allow commercial gatekeepers in front of communities to guilt users into making it easier for them to lock down and commoditize that data. It can be a difficult line to draw, but there are situations where it's worth just ripping off the band aide and saying, "if a site is not being a good steward of its community, then the site doesn't get to keep using that community's stuff as a way to attract users to the site."
No, I'm sorry, but I refuse to believe that being a doormat is less of a net-negative for the world than doing the only thing of actual consequence.
Deleting my comment history is activism against a corporations which has decided to flex its muscle in a way I don't like.
Yeah, that's the point. Users are clawing back the value they gave to Reddit because they are angry with the decisions the company has made.