That is correct if someone has the monopoly of that behaviour. But when doing that is available to everyone, there is a pesky things called "game theory" and "Nash equilibriums" who stick their fingers in the spanner.
To be clearer: the fewer people buy upvotes, the bigger the incentive to be the one that cheats. The situation is not stable and evolves toward a Nash equilibrium: people start paying more and more to bots to get on trending page - it's the system that's broken and needs fixing, not the actors who are inside the system and playing by the rules. You can indeed attempt the latter but it's self-defeating if the system stays broken.
None of which changes the fact that this behavior is in fact "stealing" reward from the users who chose not to do this behavior. None of this changes the fact that buying votes to trend a post puts the lie to "proof of brain". I love how quickly "this behavior is allowed so it is okay" is forgotten when it comes to flagging. You can't have it both ways dude. If you truly think that just because other people are going to do something, and it is "allowed" by the code, that it is okay for you to engage in a behavior that is objectively bad for the long term health of the STEEM blockchain, than it is also okay for users to flag you for this behavior. I fail to see how this is hard to understand, or even debatable. Users with large stake in this platform absolutely should be flagging the crap out of any post that buys its way to trending under the current system which does not mark such posts as advertising. Failing to do so is putting nails in the coffin of their own investment.
I fail to see how you fail to see the fatal flaw in your reasoning: who and how defines "crap" ?
If you define "crap" by "anything that buys its way to trending" then you stack the deck against any talented minnow writer who basically has no way to come to anyone's attention.
Take a look for instance at my very early posts (after the "introduce yourself") I had put a bit of work in them and while I don't think they are necessarily stellar I am pretty sure they are better than $0.00 with no votes and almost no views. I just happened to be a minnow who knew nobody else on the platform and believed the "proof of brain" theory - I was naively thinking that "good content will be discovered and will well-up by itself". It didn't.
Now look at @yallapapi. I think he had produced some distinctive content that I wouldn't have seen if he hadn't bought his way to the trending page. What he writes has value, I think, and is useful. But without buying his way to trending a lot less (and I mean A LOT LESS, like in ZERO) people would have noticed him.
He uses a mechanism not only permitted but used by many others before him in order to draw attention to his posts. Nothing unusual. Are his posts "crap" ? I wouldn't say so, on the contrary, they are a really great read. So for me defining "crap" by "anything that went to trending based on paid votes" is shortsighted and wrong. Especially since the guy paid for that advertisment with his own money.
You (or @heimindanger) are free to say that @yallapapi's content is crap but that is, in my opinion, mobbing, censorship, and basically stealing and abuse, like what grumpy cat used to do.
But if you also find that his content is actually rather outstanding then it would turn out that not every paid post that trends is crap after all ... the picture is mixed and things become a lot more complicated, like in real life. Things become debatable.
So you are entitled to your opinion that "whales should be flagging the crap out of any post that buys its way to trending" but I am also entitled to mine that this will lead to only circle-jerking whales trending for any minor "contest" that they, in their magnanimity have the kindness to organize. it will basically leave no realistic avenue for isolated minnows to get spotted for the content they produce and will drive them away from the platform.
How is that good for the long-term health of the Steem blockchain ?