I think its insanely improbable, but you can't rule it out completely i guess.
Even if finding the most opportune way to attack is very improbable, the attackers will still be able to check many different outcomes and select the best. This is not good.
But if we continue in theory, the attack is irrelevant because it can be identified long before the attackers would be able to hurt the honest SHs. If Walmart's cartel was controlling a large portion of consensus, everyone would know that Kmart's transaction verification is very slow. Kmart would not be wise to keep something hurting its business a secret.
Since the code that CNPs use to drop TBs is client-side, it could be modified to start being more aware of SHs intentionally dropping valid transactions (perhaps this is something that should be included from the start). Care has to be taken with this though. In the weakest prevention scenario using this type of defense, at the very least Walmart's transactions would start taking longer to approve, and Walmart must start being honest again or it will be in a ridiculous situation.
edit: I mixed up two different attacks there, since there are sort of actually 3 attacks that have been recently discussed and the third is a sort of combination of the other two. It's all starting to run together at this point. Decrits is not weak vs. any of them, though yes a few shenanigans are possible given an entity controls a *very* large portion of consensus. But I don't expect perfection from Decrits, and that is why rather than creating a *crits clone to fix an identified problem, the network could instead adapt, or worst-case split peacefully with each ideal going separate ways. I have some really neat ideas for section 4, but fleshing out 1, 2, and 3 is much more important for now.