The implementation of vote decay in Dawn 4.0 has been introduced to tweak the system contract.
Does it prevent the use of automated voting bots for the purpose of vote-decay?
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Vote decay is not intended to prevent the use of bots.
The purpose of vote decay is to stop a block producer from becoming 'lazy', in the situation where they get lots of votes from people who then stop using EOS.
Imagine if someone with a large amount of tokens voted for a block producer, effectively guaranteeing that the block producer is in the top 21. Then the voter somehow loses their private key and their tokens are lost forever. If the block producer were to realize this, then they lose incentive to work hard on producing and validating blocks, because they can't be voted out anyway.
If the votes decay over time, then the original votes from the person who lost their private key become worthless after 2 years, stopping the block producer from taking advantage of the situation.