Paid option doesn't really deter this behavior, it encourages it - a botter will see a price tag on a "real" account (see what happened to twitter's blue checkmark sub) and go oh goody, I can pay for people to think I'm real.
If you make the price high enough sure, but I'm unsure you can find the right price to simultaneously 1) deter bot traffic and 2) be appealing to actual users.
in other words, it just becomes the cost of doing business.
the individual user is now priced out and cannot speak candidly and anonymously, while large, wealthy orgs simply price that into their market-capture and consensus-building techniques
I'm trying to imagine this new paid app in different angles and versions, ie a new Reddit... Pay to be in there, get paid for being in there, only humans can be in there, ads pay for humans being there, humans use some govt on-line ID system, karma systems improve so that only humans are rewarded, Voight-Kampf captchas, humans mail the app their dna to verify their identity, humans login on a street 24/7 login post (think phone booths).... I just don't see any good, unbreakable, viable and/or sustainable way. We just need to get used to coexist with bots everywhere while we adjust our expectations and social codes. Fast forward until AI is massively on the streets and indistinguishable from us physically (or very distinct and fascinating), and all supposing that we can keep them in control...
Dead internet is the prequel to dead world, let's seize the opportunity to learn how to coexist with synthetics and develop the code that will make life with a higher intelligence species possible on Earth. And remember, we humans vary widely, and just like there are people happy to share LinkedIn slop today, there will be humans gladly living surrounded exclusively by overpowering synthetics. So lower your expectations for universal solutions and focus on niche.
If you make the price high enough sure, but I'm unsure you can find the right price to simultaneously 1) deter bot traffic and 2) be appealing to actual users.