I don't see how it would hurt honest users: even my best friends I don't need to upvote several times per day (or even every day) ... :) And I still could do that, if I insisted to, I just had to accept somewhat lower returns.
For example the curation window hurts me much more than diminishing returns ever could, as I never try to upvote any post within the first five minutes after it has been published. I can't find good articles that fast if curating manually (only auto voters can) and even if I did, I couldn't read and evaluate them within five minutes. As a result nearly everybody with a comparable STEEM power earns more curation rewards than I do ... but the good thing is: I don't care that much. :)
Anyway: you prefer the current state to solve the problem by downvoting abusers it seems?
Well no, I don't. But at least it's kinda visible when they stick to the same account and don't constantly switch through several. It's not really a solution, because it would hide the abuse even better when someone decides to do it.