Few things I disagree with, Regidrago being good into every slow fat team w/out a fairy is no longer true once you factor in tera. Specs tera steel tera blast will destroy fairies, and keeping dragon stab+dragons maw as ur ability means dragon energy is still destroying every non fairy. U also become a pure steel type with 200/50/50 bulk which is no joke. I think this mon will be far from bad once u allow it to actually break past fairies.
Glastrier will also get to change its shitty pure ice typing into smth better like pure fighting+CC or pure steel+heavy slam or hell I can see tera water/fairy being fine too. It will get to leverage its godsent stat distribution of 100/145/130/65/110/30 much more effectively with a different typing, but I'm sure it won't be as broken as others.
Volcanion under rain was already borderline impossible to switch into, throw in tera water into the mix and now it's even more disgusting. Think gastro is safe? Tera grass tera blast will catch you there too. There's honestly a solid case for this shit becoming crazy.
It's really funny that you're comparing tera blast+tera to pissweak moves like shadow claw and hp ice. Tera blast+tera ghost on a physical dragapult is a 80*2=160 base power move while shadow claw without tera is just 70*1.5=105. Likewise, tera blast+tera ice on regieleki is an 80*1.5=120 base power ice move while hidden power is just 60. Let's be real here, these are busted because tera is busted.
Espathra's in the same boat, it is laughable to think it would be broken with hidden power. Let me show you some calcs to show you the difference that tera makes:
+1 0 SpA Espathra Tera Blast vs. 0 HP / 0 SpD Chi-Yu: 222-264 (88.4 - 105.1%) (tera fighting+tera blast)
+1 0 SpA Espathra Hidden Power Fighting vs. 0 HP / 0 SpD Chi-Yu: 112-134 (44.6 - 53.3%) (no tera, just hidden power)
I've said it already, but espathra is MID without tera, and tera is the only factor pushing it over the edge.
I feel like a lot of these "nicher Pokemon become broke because Tera" theories have the problem of, well.
Let's assume Glastrier as Steel type is broken. I don't know really, but let's assume it. You are now running an Untiered Pokemon, a slow Mono Ice-type with good stats, but many crippling weaknesses...
So, we make a Tera Steel set. This is a good set, but two problems:
1. Switching it in is not hard per se throughout a game, but a lot of its problems competitively is switching in, taking a hit after setup or using a good move, and then likely being revenged at best.
2. You have used your Terastilization. If you do not use Terastilization on Glastrier in any given game, you are running the tier equivalent to Glaceon.
While there are many Pokemon in the metagame that are safe Terastilizations, a lot of them are on Pokemon that, arguably, are still good without it. Skeledirge has pretty good stats, and a moveset to take advantage of it. The ability to Tera Fairy moreso is a complimentary choice- Fairy checks most of what Fire doesn't, or simply isn't weak (Not weak to Ghost, Ground, Water, Dark, Rock). But Skeledirge would still be pretty good even without Tera- maybe UU or at worst RU by my opinion, though honestly as the metagame dies down, even with Tera I can see that happening.
Glastrier though? It's... not even PU by usage last gen. It's not a good Pokemon (in Singles, VGC overlords please do not skin me alive!), and to make it good (ie. in most competitive game terms, "activate it" as a part of the team) you are losing more flexibility and using up your Tera.
Do I think all of these sorts of cases will lead to a result of "Not worth it to run"? No, but not of Pokemon like Glastrier.
On your other examples: I disagree about Regidrago due to the things above (I'd rather run Wish+Pass Sylveon personally, it's actually pretty good), Volcanion is definitely a Pokemon that could be a bit much.
In general, Rain is going to be crazy with Home. Tera Water boosted Specs Ash-Greninja Hydro Pump and Volcanion is crazy.
Though honestly, Ash-Greninja would probably be banned anyways. Volcanion is a pretty good case to make.
Regieleki is going to be broken with Terastilization. Unless we all want to run Breloom/Accelerock Lycanroc/Lucario every single game, and use them super sparingly in order to not lose.
Even if Volcanion was broken by Tera, and Regieleki will be broken via Tera, as I've made clear in prior posts: I care more about mechanics than Pokemon, as long as the mechanics are good. I know you disagree with this, but just adding my opinion on that.