Well I think vote selling will obviously be much more lucrative than curation rewards since the max you can get with curation is 25% and you need time to find proper posts to curate, while with vote selling you don't need time and receive close to 100% of what you are voting (in the case you mentioned -15% so you get close to 85%).

The only problem with that, is that organic curation will be lower and newer users will find it harder to receive support, which perhaps might affect the long time growth of the platform.

Imagine if every user does this, how could a minnow with 0 investments find support and grow his/her account? It would be impossible.

Instead of "write blogs, get paid with steem" we would have "pay bid bots, get spam comments" lol

I think if this dynamic really takes over, this platform will sadly die slowly but surely, although since there is no competition it would take a few months perhaps a few years, but a place in which you need to buy support instead of earning it will never be sustainable.