Yes, they can look back and say "man, we whiffed here, that guy ended up being amazing," but they're not saying "the Steelers selected Antonio Brown with the fifth round pick they received from the Skelton trade, so that means we would have had Antonio Brown in the fifth," they're saying "we really screwed up on our evaluation of Antonio Brown. We should have definitely selected him instead of Andre Roberts in the third, or even over Dan Williams in the first." Even if we still had our original pick, we probably would not have selected Antonio Brown, we probably would have selected someone else on our board, without the foresight of knowing that Brown was going to become one of the NFL's best.
The evaluation of a trade like that can really only be correlated to your draft board, just like the Texans are probably kicking themselves that they don't have access to Saquon Barkley right now. But Barkley could turn out to have a drug habit and only play in 5 games, which wouldn't make the trade for Watson any more or less successful.