I have no numbers to back me up, but my impression is that the majority of Reddit’s revenue comes from people browsing recently submitted posts, where they can inject ads.
Old posts/comments with technical or insightful information are surfaced via other search engines, not via browsing, so Reddit makes virtually no money from them.
Deleting these posts and comments won’t hurt Reddit, only the people seeking information.
it hurts reddit's value prop and is suggested in addition to stopping posting new content, not just to cull content once it gets old. your worry about old content going away should be in proportion to how much new content degrades/slows.
It’s a bit like those stack overflows that end with “never mind, figured it out” without the actual answer.
I’ve encountered that multiple times on reddit where people scrubbing their history and it breaking the conversation enough to be useless