But I don’t think you’re forbidden from bypassing HP simply because HP exists.
HP are a mechanism the game presents for resolving uncertainty. Was so-and-so killed by such-and-such? Yes/No: narrate it. Not certain: determine damage and compare to remaining hps.
Saying that's "bypassing hps" is doing it out of order. Hit points only come into it if you're not sure they're dead.
It's like resolving any other action declaration. Sometimes you just don't call for a check. If a player declares "I waste him with my crossbow." Then the DM narrating "He falls to the floor twitching and bleeding for a few moments, then is still," is as valid as "Roll to hit."
How would the player know that? I mean, to begin with, how did you the DM know that a pointed crossbow didn't inhibit or prevent an effective defense?
The DM exercised judgement, the player would only know it if he asked took some action to determine if it were true or not (possibly a mental action, like, "in my years of military service, did I ever see...").
Suppose that the pointed crossbow was pushed up against the PC's back? Would that now "inhibit or prevent" an effective defense?
IIRC, it'd realistically inhibit or prevent an effective attack with the crossbow. Even 5e-RaW (contradiction in terms that may be), it'd be with disadvantage.
You did say, "swarms of ankle-biters like goblins and kobolds who might try to overwhelm and drag you down." How many is a swarm, that I might know ahead of time as a player when I'm automatically doomed and have no defense?
A couple hundred, I think constituted a swarm in eds that had 'em, and a swarm might well have an attack that engulfed an enemy.
Why would anyone make normal attacks when they could bypass hitpoints in this manner?
Because they failed to sell it as a sure thing, so are relegated to using the mechanics, perhaps. But, yes, bypassing a game's main defense or plot-armor surrogate gets very dangerous to play. You can easily get a degenerate case where a major feature of the game is just gone in a practical sense.
So basically, a party could pretty reliably hack 1-2 limbs off the average foe in any one given round. Yep, that qualifies as a go-to tactic. Chop the fighter's legs off and finish him from range, or chop the wizard's arms off and face his mighty arcane headbutts.
Bigby's Mighty Arcane Headbutts
Evocation
Level: 5
Casting time: 1 Action
Range: 120 feet
Components: V
Duration: Concentration, up to 1 minute
What do you call a mage with no arms and no legs? Dangerous.
You create a Large disembodied head of shimmering, translucent force in an unoccupied space that you can see within range. The head lasts for the spell’s duration, and it moves at your command, mimicking the movements of your own head.
The head is an object that has AC 20 and hit points equal to your hit point maximum. If it drops to 0 hit points, the spell ends. It has a Strength of 26 (+8) and a Dexterity of 10 (+0). The head doesn’t fill its space.
When you cast the spell and as a bonus action on your subsequent turns, you can move the head up to 60 feet and then cause one of the following effects with it:
Mighty Headbutt
The head strikes one creature or object within 5 feet of it. Make a melee spell attack for the head using your game statistics. On a hit, the target takes 4d8 force damage.
At Loggerheads
The Head attempts to push a creature within 5 feet of it in a direction you choose. Make a check with the Head's Strength contested by the Strength (Athletics) check of the target. If the target is Medium or smaller, you have advantage on the check. If you succeed, the Head pushes the target up to 5 feet plus a number of feet equal to five times your spellcasting ability modifier. The Head moves with the target to remain within 5 feet of it.
Clenched Jaw
The Head attempts to grapple a Huge or smaller creature within 5 feet of it. You use the Head’s Strength score to resolve the grapple. If the target is Medium or smaller, you have advantage on the check. While the head is grappling the target, you can use a bonus action to have the head chew it. When you do so, the target takes bludgeoning damage equal to 2d6 + your spellcasting ability modifier.
Head Him Off
The Head interposes itself between you and a creature you choose until you give the head a different command. The Head moves to stay between you and the target, providing you with half cover against the target. The target can’t move through the Head’s space if its Strength score is less than or equal to the Head's Strength score. If its Strength score is higher than the Head’s Strength score, the target can move toward you through the Head’s space, but that space is difficult terrain for the target.