You make some very valid and reasonable points here. The issue is not so much the shitposting in itself, it's the whales with high amounts of Steem power shitposting and then self-upvoting it or using paid voting bots to get to the trending page.
I think if these whales shitposted and didn't self-upvote, people would be surprised to see these posts would be valued accordingly and make nothing. It would not be an issue personally, the bidbots are a whole other kettle of fish.
I wholeheartedly believe self-voting should be significantly handicapped, make a self-vote worth half of what it currently is and incentivise voting up other content (increasing curation rewards). I think you would see better content rise to the top.
There will always be shitposting, but I am a strong believer a shitpost is not worth anything more than a few dollars, most definitely not worth $40+ like some of the posts I have been seeing.