How will that help? The seller can still influence and finance the buyers. It may be more expensive but I'm certain the seller will do what is necessary for those high rankings. The stakes are that high.
If Amazon sells 10,000 Acme Widgets and sends an email to just 1% of those customers asking for a review. How is Acme going to game that? Pay all 10,000 purchasers of their product so that the 1% that get the Golden Ticket to write a review give them 5 stars?
The problem seems to me to be that the reviewer gets to decide to post a review. Turn it around and have Amazon select reviewers at random and you've made it much harder to game.
I'm not in the industry but I'm willing to bet there are flourishing communities that participate in this type of trade. And it's as sophisticated as it needs to be to get around Amazons countermeasures.
How else are we seeing so many corrupt reviews? Not like Amazon is just passively watching this happen. This is a huge issue for them since it's a bad user experience and Jeff Bezos is obsessed with satisfying customers.