You need to learn what subjective and objective mean. An officer testifying about all his training and experience is done specifically to add validation to his decisions and opinions. These are by their nature subjective.First off, the law says it's illegal to drive under the influence. It also says it's illegal to drive above .08. These two provisions operate entirely independently. There is no current requirement a police officer attempt to administer a breathalyzer to effect a DUI arrest, nor is there a requirement that someone who has taken a breathalyzer register above a .08 in order to be arrested for DUI.
So this parade of horribles you're suggesting where an officer arrests you because you walk in a wobbly manner is already theoretically possible. But it doesn't happen because it's also illegal to arrest without probable cause. The presence or absence of the option to arrest based on a canonical numerical threshold neither adds nor eliminates common sense from the equation. I would argue that it actually adds arbitrariness in that it allows a police officer to effect an arrest upon an innocent person who is clearly in complete control of their faculties just because they register a certain number on an instrument.
Secondly, no one is advocating for the elimination of breathalyzers. No one is advocating for a non-objective or opinion-based standard. Behavioral patterns are objective. All of the criteria I listed are objective. None of them are necessary and none alone is sufficient. A police officer can only arrest if based on the totality of the circumstances, there is sufficient probable cause that someone is operating a vehicle under the influence. In addition to the potential for video evidence, I listed "slurring your words, unbalanced on your feet, swaying back and forth, aggressive, and smelled of alcohol" as potential objective indicators of intoxication, as well as "all over the road, driving twice the speed limit, and/or crashed your car" as ancillary factors to be considered. There are many, many other objective fact-based considerations for a police officer to take into account.
We have an adversarial justice system grounded in facts and evidence. A police officer does not come to court and say a defendant "seemed drunk". Every defense counsel in America would object to a remark like that and every judge in America would strike it from the record (or be overturned on appeal if he didn't). An officer explains the facts of the defendant's behavior and presents the circumstances and the evidence. He explains his training and why those behaviors are factual indicators of intoxication. The defense counsel can cross-examine and present his own witnesses. The judge explains the law. The jury decides the truth.
Third, before the strict "per se" numerical threshold, breathalyzers still existed, and they were still used in court. But they were neither necessary nor sufficient for an arrest. One of the older laws in MA read that a BAC reading below x% could be used by a defendant as evidence they were not under the influence of alcohol, a BAC reading between x% and y% meant that the reading could not be used as evidence in either direction, and a BAC reading above y% was evidence that they were intoxicated. That's a perfectly reasonable evidentiary rule that allows BAC evidence to be considered alongside any other evidence that's available.
Finally, take a second to think about any other criminal law. In a murder trial, even if there's DNA evidence, it's evidence, to be considered alongside a whole lot of other evidence. Virtually no laws have a canonical scientific test to prove guilt. Crimes and criminal law simply don't work that way.
For whatever reason people love the simplicity of the idea of DUI BAC thresholds, and are entranced with the idea that they're 'objective' and 'scientific' while the alternatives are not. But it's all pure applesauce. It's a narrative that's been beaten into our heads over the years by the lunatics at MADD and their statist Congressional enablers. Not to mention the joint media/MADD collusion to use the emotions of grieving parents to produce "this bill will bring my dead kid back and if you vote against it you want more dead kids" laws like Melanie's law.
There's no single test that proves guilt for theft, arson, fraud, negligent driving, assault, harassment, forgery, extortion, kidnapping, homicide, prostitution, rape, perjury, etc. Arrests are effected based on probable cause. You have an adversarial trial where both sides present objective facts and evidence. The judge explains the law to the jury. The jury decides the truth. That's the way it works. That's the way it ought to work with DUI.
Slurred speech is a perfect example, what you consider slurred, what I consider slurred, and what is slurred to an individual are all different and a matter of our subjective opinions. Or do you know of a scientific means to measure slurred that does not consist of an observers opinion.