One option is to keep the base experience free, with certain features included in a subscription model. If balanced properly it allows for both open use and funding.

The main hurdle is user adoption. Only a small percentage of a user base will pay a monthly fee. 

 Payment is tied to identity, meta data at the mimimum.  People don't want that.  They want privacy and control. If nostr takes that route users might as well stick with twitter and hand over their cash to elon. 

 When the base experience is free there is no need to pay. I don't pay for twitter, yet some choose to pay for additional features.