Fundamentally, karma limits were mostly used as an anti-spam measure, if a rather crude one. I was actually a mod of a couple relatively large subs, and while I did feel bad for the impact on new users, the benefit from how much spam we caught with it made it ultimately worthwhile. There very well may be better approaches out there though, and I am excited to see how things grow here, but it is going to be a problem that the Fediverse will face as well.