I agree with what you say with some distinctions.
By spreading the stake across multiple accounts and voting for the same witnesses, whales have accomplished nothing in terms of influence. It's the same. They may conceal this way the fact it's coming from the same account though.
If they do it to vote for different witnesses, then the influence spreads thin, and they can be overthrown by smaller votes acting together.
If consensus remains of 21 and number of votes per account diminishes to 5 or 10, one account cannot 'control' as many witnesses, unless I'm missing something. It may be a good measure.
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