I think you are misinterpreting some elemental things here.
The rewards are not and will not be 50% of your SP per year. I don't know the exact ROI you can get by voting only yourself but I think it's around 18% or something like that.
And after the possible change to a 50/50 reward system your ROI won't skyrocket to 50%, it just means that the curation rewards will be 50% from upvoted value, 50% for the author and 50% curation. The ROI will still be lower than 50% and that by a lot xD
And sure there will be more downvotes, if ppl see something they don't like it's very possible that they will downvote it after the change and I think that is good because now a lot of trash doesn't get doenvoted because it costs money to downvote and for most ppl it's that they are more generous with their upvotes instead of downvoting something.
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With the current 75/25 payout the ROI per year may be 18%. If it was 50/50 per year it would be between 36% and possibly 50%. You get more if you upvote something before others, and within a certain time window.
It couldbe possible to get a 50% return.
If anything it would be a doubling from 18% to 36% then lets say. That is still quite a chunk of money. There are many people in the crypto world who could buy 200,000 USD of Steem, upvote any random posts just to make money. That would earn them 72,000 USD a year for no effort.
A no effort community filled with people buying Steem just to make profit would not hold up in the long term.
Giving free downvotes to everyone will result in more downvoting, not smarter downvoting or better curating. I would be willing to vote for "mods" or "a council" who actually gets the free downvotes. Those people could be voted for just like how witnesses are voted for.