You’re right. There will need to be a way for new users to earn trust. 

I can think of a bunch of options:
1) pay influencers to sponsor them
2) pay relays to post
3) proof of work to generate a npub or to post to relays. 

I’m sure there are other options but these are just the ones that come to mind quickly. 

Bottom line is that there must be a real cost to prevent spam.