I agree the majority of the value is in the current activity but theres definitely some in the historical data, both for AI training (as you mentioned) and for people looking for information on topics that have been discussed in the past.
My assumption though is Reddit has already done the math regarding the threat of reduced value as a knowledge repository from people deleting comments/posts and decided the value they lose from pissing people off is smaller than what they gain from the changes that everyone is angry about.
Especially because they probably keep backups they can pull from and (as people have pointed out) historical post/comment data is already archived and available all over the internet. Legally that data is still the property of Reddit so if AI companies want to use it without breaking laws they'll need to pay them for it.