First and foremost, lowering the drinking age from 21 to 18 (in line with the rest of the world, and a good chunk of American history) is NOT a radical change at all. So any (real or imagined) fears related to lowering it even further below 18, or abolishing the age limit altogether, do NOT apply to lowering it to 18.
Secondly, it is NOT like the (real or imagined) fears of suddenly legalizing something that was previously completely illegal for everyone. Alcohol in its various forms has been broadly legal, tolerated, and openly advertised for most of recorded history. It is the "devil we already know", basically. So "same as before, but without the arrests or injustices" is actually pretty close to accurate here.
Thirdly, simple and practical things like raising the alcohol taxes and cracking down harder on drunk driving, drunk violence, drunk and disorderly conduct, and stuff like that, would basically render any feared unintended consequences academic in practice. This is true whether the drinking age is lowered to 18 gradually OR suddenly.
Fourthly, it's not the 1970s or 1980s anymore. America is almost a completely different country now in so many ways, including for drunk driving. The original, albeit still fatally flawed, crisis-based justification for coercively raising the drinking age to 21 in the 1980s is itself grossly outdated and basically academic now in the 21st century, hence the need for the pro-21 crowd to invent new, ever more specious and spurious justifications (such as junk neuroscience, and a perversion of the precautionary principle) for keeping it at 21, because reasons.
And finally, it is ultimately a matter of principle, as it is fundamentally a civil rights issue, far more so than it is a public health and safety issue. Not everything can be reduced to crude utilitarianism, after all. "Let us do evil, that good may come" is always and everywhere a trap, a lesson that is unfortunately too often learned the hard way.
(Mic drop)
