>considering the default prune behaviour of imgboards

One of the parameters Kareha uses to determine pruning is MAX_POSTS, so even if you sage a thread under this new condition, you still add to the board's total postcount and speed up the process for pruning that thread, regardless of MAX_RES. The only flew is this assumes pruning is based on thread creation date, not popularity (because someone could easily bump a shitty thread and save it from deletion).

I also agree that enabling this functionality would further make threads vulnerable to intentional bumping by trolls. I was sorta envisioning it being used in a mature community where trolling is minimal and quickly weeded out by regulars.

Here's a new idea: how about trying this in reverse? Only "sage" posts are counted in MAX_RES, in which case saging can again be used either in protest or as a courtesy to others. The only problem is that people can then freely bump threads without consequence.