The metric the agent's boss's boss's boss cares about is convictions because that's how they justify their budget "look at all these criminals we're catching". If they can't use the illegal searches in order to convict people of crimes they're much less likely to spend the man hours doing them as routine procedure.
Also, it's not exactly hard for a lawyer to argue that you were coerced into consenting to a search in your hypothetical example. Since the people crossing the border are mostly non-criminals if CPB does use those tactics routinely it will not take long at all for a suitably politically sympathetic victim to come along and precedent to be established saying that is also not ok.
Alas, a metric they just don’t care about is passenger feelings. The power tripping is real, and most people would consider being detained for hours to be a big issue, even if it never even goes to court.
> If they can't use the illegal searches in order to convict people of crimes
I highly doubt they are getting a lot of convictions from illegal searches of devices. The overwhelming majority of convictions they get are from people physically smuggling drugs and weapons and apples across borders. They don't need to unlock your phone to catch you with any of these things. They already have the right to tear apart your car if they think you have drugs hidden in it.
It's a power play, not an actually useful LE tool. The overwhelming majority of the things that you may have on your phone that are going to get you in trouble with border control are not going to put you in jail. What they will do, is deny you entry, and possibly ban you for 5-10 years.
Since entry for non-citzens is a courtesy, you have zero redress in that case. You're never going to get your day in court, since you're not charged with a crime, and you will not be able to contest that the search was illegal.
The only way this ruling would result in any change, is if CBP voluntarily follows it.
Also, it's not exactly hard for a lawyer to argue that you were coerced into consenting to a search in your hypothetical example. Since the people crossing the border are mostly non-criminals if CPB does use those tactics routinely it will not take long at all for a suitably politically sympathetic victim to come along and precedent to be established saying that is also not ok.