There's absolutely nothing invalid about your concerns. You could have said, "without a cap on the debuff stack, this could be game breaking" "without reducing the duration of the rebuff, it would be problematic"
Yeah, but it wouldn't have been as entertaining.
Let's just clear this up: Is the change "needed"? No.
Needed is such a useless word. If there were glaring gaps in a class or spec's toolkit, it would be addressed in some
way; sure it's not needed. Whatever.
The points to be made are really:
1) Would it be fun?
2) Would it make a playstyle difference... which is fun? (leading back into #1 here)
3) Would it be balanced?
4) Is it easy or hard to balance?
By virtue of bears being in a pretty damn good spot right now, you already throw out #3. #2, well, damn right it'd make a difference to making TaC like ShoR in that it's an almost reactionary-proactive ability (since it's consumed so fast). But is that a good thing - is it fun? Well, not necessarily. First off, since it scales with Vengeance, you do run into that exact thing where it's like "so I'm offtanking and want to lower Snapping Bites for my co-tank, here's my... 20k reduction?" Worthless.
Secondly, since TaC works for anyone the boss hits, not just the Druid who applied it, you automatically get thrown into "off-tank" duty. As a Warrior in Cata I can tell you OFFTANK DUTY FRICKIN' BLOWS. There are some few fights it's fun on, but the majority of fights you go "UGGH I AM SO USELESS I STAND HERE AND TAKE HITS." Tank damage? Nope, you're offtanking. Tank mitigation? Nah, boss always hits harder. Tank.. raid utility? Yeah, like you're going to HotW tranq while tanking mobs.
Basically on top of all this, you have condition #4. Now think of all the caveats that you have already thrown in (have to nerf stack size, have to nerf potential duration) and... you end up with a weaker product than now, against auto-attacks. Well, crap, that ain't even a buff, it's a nerf. Not only that, because TaC works against auto attacks as well, it's gonna be pretty much impossible to time against non-casted boss specials such as Triple Puncture or Overwhelming Assault. Hell, it's already annoying enough to time ShoR for OA, you want to do it with Maul while the timer is off AND you want to avoid melees at the same time? The only saving grace is that at least you have high avoidance with SD.
Ultimately, I think the Guardian AM toolkit (their whole toolkit in general, including Mastery) compared to the other tanking specs in this expansion is very much an experiment. Not in a bad way, mind; just because it's an experiment doesn't mean Blizzard just tosses it out to dry. I mean, DK tanks in all of Wrath were an experiment, and they continued to be throughout Cata (with ups and downs appropriately). It's not a RISKY experiment though; since Blizzard loaded bears with EH, it's seriously not like (outside of 25H MAYBE) things will just start one shotting you all over the place.
And honestly I want the consensus from Blizzard to be, "okay, people like RELIABLE AM; we should change Guardian AM next time to be reliable, but preserve their flavor in high avoidance and EH in some other way." I don't know if that's what most Druid players think, but that's what I do. But honestly, no way to know how that's going to pan out until next expansion.
Basically to sum up, what I'm pointing out is that while the concept
of having a proactive ability to mitigate boss specials is nice, it doesn't work right now, and especially not with TaC. It won't work in a hotfix because it'll take a TON of changes to, honestly, a fairly minor mechanic, and change it to either a very major one (on some fights) to an uncontrollably RNG one (depending on how lucky you are with dodges/hitting it in time for noncasted specials). That's a really big change for just a patch.