Even commenting and voting can earn sats (2/3rds of the daily drop now comes from those methods).

Costs 1 sat to comment, and you might earn 10X or 100X just from tips on that, and might also earn from the daily drop if the criteria for the algo for the daily drop was met. Upvoting is 1 sat as well, and might yield you more than the 1 sat in return.

i.e., many people who contribute will not ever need to top up.

Where do the funds come from? Well, for now, it's coming from the site's revenue. After some time, when that changes, those funds come from the users who are fine with topping up their SN account to be able to continue participating.

Which will work better? Who knows, this is an experiment in incentives, network effects, etc. All I can say is I used to spend a LOT of time on r/bitcoin. I haven't logged in there in months.