A "better-than-the-average-bear" is already ripping a lot of other big nasty monsters to pieces. I see absolutely no meaningful distinction there.
You are aware that ghosts are
immaterial, right? I don't care how buff Yogi is, he can't sink his claws into something that physically isn't there.
It's reinventing the wheel as relates strictly to 5e. Every other class that uses physical attacks has one way or another to bypass DR/magical permanently (i.e. finding a magic weapon, or being a Moon Druid or Monk).
If finding a magic item counts, why does the ranger (or the monk or the druid) need a class feature? They can just find a magic item to do it.
And it was a terrible solution in 3e.
Why? I've already explained how it enhanced the game by making magic DR an obstacle that must be overcome proactively.
Resistant monsters are pretty common at higher CRs. Yes, it absolutely would be a spell tax.
While this was certainly true in earlier editions, it's much less so in 5E. The game was very deliberately designed to be magic-item-agnostic. When you can kill an ancient red dragon with an ordinary longsword straight out of the PHB, I simply don't buy the "magic is obligatory" argument.
And even when magic
is obligatory,
that's a bigger issue with the system. It creates precisely the problem we are both (from opposite ends) outlining: a rat-race where the monsters get a defense that the PCs are obligated to receive a counter to, with a net gameplay outcome of zero. That's
why 5E is very deliberately designed to be magic-item-agnostic.
Comparing to Blight is patently ridiculous. Creatures with DR/magical are a much more broad category than specifically plant creatures. Moreover, without a means to bypass DR/magical, you're pretty much crippled in those fights, whereas there's plenty of ways to kill plant creatures without ever having to touch Blight.
What's wrong with being "pretty much crippled" in some fights? If every fight were exactly the same difficulty and could be solved with exactly the same approach, this would be a much less interesting game. Spellcasters are "pretty much crippled" against creatures with spell resistance, and they don't even get any spell in 5E to punch through it. If they did get such a spell, would that be a spell tax too? If so, then I guess it's good for the game that they don't -- and this seems also to be an argument against letting beastmasters even get
magic fang, much less a permanent effect.
You're objecting to literally ONE more subclass having the same ability as the Moon Druid or the Monk. A subclass which needs such an ability for the sake of game balance, because said subclass gets much of their offense through their pet rather than their weapon. That hardly amounts to "wasted text in the monster statblock." That's a slippery slope fallacy to the extreme.
As I said, if you're invoking balance to justify this ability, then the balance is out of whack to begin with. Furthermore, if you're invoking balance to justify this ability, then you're
not invoking in-universe logic or the realization of class concept. Characters should receive abilities because they are
fun and
exciting, not because they have to.
And I don't think you understand what I meant by wasted space. Look at what's happening in the abstract. Say that all or most monsters above level X have the "Lock" ability which stops them from being damaged except by stuff with the "Key" keyword. As a result, all PCs automatically receive the "Key" keyword at level X which lets them damage the monsters normally. What is this Lock/Key mechanic doing for the game? Nothing. It cancels itself out. Gameplay is exactly the same as it would have been if the mechanic didn't exist. It is wasted text.
You're trying to argue that weapon resistance works like this Lock/Key mechanic. I'm trying to argue that, fortunately for the game, it isn't. Some monsters have Locks, yes, but getting a Key is an interesting challenge rather than something you're just entitled to by virtue of system math, and if you don't have it, the Lock can usually be overcome another way anyway.