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No, they won't sign as often, that's the point. They are backups. The consensus witnesses need to be the most trusted. If every witness signed as often, it would be really easy to sybil attack the chain.

OK. Makes sense. But couldn't other witnesses become consensus witnesses over time as they prove their trusworthiness and thus increase decentralisation. 20 seems a small number from a dentralised perspective.

If you add more, you slow down the chain, although the recent one block feature helps this a lot, but increasing it also increases risks.

Interesting. Thanks for answering my questions.