Sadly yes, and this logic also applies to payments, so those saying micropayments of some kind to take in the network part fixes this are not entirely correct. Spam is an industry. Dedicated spammers will always earn from their spam, be it via simple impressions, or as a front door to some scam or another, or taking payment to bring someone else's system down, and so on. And it's the dedicated spammers you need to keep out, they will always appear at some point, typically once the impression base seems big enough. However the amount dedicated spammers can earn from spamming the network is almost always more than what a user in a less developed country can pay to take part in the same network. And sometimes in a more developed country too. So you end up at the same failure mode.