It would have been better for everyone--Amazon, authors, readers--if they'd never allowed sponsored books at all, or at least kept them below and perhaps smaller than the real also-boughts.
Some may see this as heresy for even suggesting it, but frankly, if Amazon wanted to make more money off KDP, they'd do better by (say) lowering the royalty rate to 65%, or charging a "per-month, per-book" fee to be on KDP--if that would keep the neutral recommendation engine in place over the pay-to-be-seen. The heavier the influence of ads, the more of a Wild West it becomes and the less the customer benefits, IMO.
"Money changes everything." --Cyndi Lauper/Tom Gray