Hi Lukasz! Welcome to Couchers and our community forum. 
Let me share some thoughts on questions you’ve asked. I’m one of the co-founders of the project, and I have oversight over a lot of our core contributor teams.
Also, if you have more questions like this or want to meet some of the team, please join us on Sunday at the Open Town Hall which is specifically for this kind of stuff!
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We have gone with a voluntary donations model for now. We absolutely want this to grow and have millions of users, and that certainly requires large amounts of money. The bare minimum is paying for admin costs (legal, accounting, etc); and for servers. The former is basically a constant yearly cost, the former grows somewhat sublinearly with users. The cost of admin should be around $1k per year, and the cost of servers is around 5-20c per active user per year. That money will be very easy to raise through donations (every 100th person would need to donate $20). However, of course we want to do more than the bare minimum, and we’d love to raise more money. We believe we can raise a lot more through donations if we are open to it (some projects reject donations beyond the minimum), do a good job, are transparent, and show people the money is well spent. We don’t want to run ads: that just makes the experience worse and the way ads are done on the web these days is not respectful of the users. We also don’t want to create different classes of users, and an expectation of better “service” from paid users.
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If we get enough money, we would love to spend it on enhancing our local Couchers communities, and helping them operate, run events, and do outreach. For donations though, we should probably spend the money on the states purpose: advancing couch surfing, building and maintaining the platform, and encouraging related activities. So not veering too far in our expenditure.
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This is something that we’ve been thinking about a lot both among the core contributors as well as here on the forum. I do not think inactive users need to be deleted: if someone can’t host for two years while living in a tiny apartment, there’s no need to delete their account, references, etc. It’s sufficient to hide their account from search and listing, etc.