Why is it always, exclusively, portrayed from the standpoint of the player who didn't use the game-winning terastalization?
If your opponent has a Kingambit they're preserving, and you have a healthy Zamazenta, and they aren't doing everything in their power to force it in to expend Dauntless Shield and accumulate some chip, you should be expecting either Tera Flying/Fairy with Terablast, or Tera Ghost. From the moment they decided that was their game plan, they had to focus on removing other checks, preserving their use of terastalization, and not allow Kingambit to be forced in and take chip early. If they pull that off and win, why shouldn't they win? You were outplayed!
Tera Dark isn't "fishing" for Dondozo, it's packing coverage for an otherwise-counter; the coverage is just its tera type, rather than a move slot. If I slot Wisp on a Dragapult, is that "fishing" for Kingambit? What if I pack Play Rough on an Ogerpon-W to ensure dragons aren't safe checks?
Fishing requires something to be meaningful sub-optimal normally, but excel hugely in a specific matchup. Simple good options aren't a fish, unless you want literally every coverage move to qualify.
I appreciate the reply. I had a feeling this would be brought up. I was thinking about it a lot and came to this conclusion:
Tera is inherently matchup fishy. Here's a better (and more realistic) example. Let's say you are running a team with two Gambit checks in Zamazenta and Dondozo. Your opponent's strategy is to bruteforce your physical walls to make way for a Gambit sweep. They are putting a lot of offensive pressure on you with Gambit's physical teammates. Because of this, you can only save one healthy mon for the end to check Gambit. Do you save Zama or Dozo?
If your opponent is running Tera Ghost Gambit, they win in the scenario where you save your Zama. They lose if you save your Dondozo. On the other hand, if they are Tera Dark, they win against Dozo and lose against Zama.
Since you cannot adequately predict the Tera, the outcome of the game is reduced to a flip that is outside of either player's ability to strategize.
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That was a simple example that illustrates the uncompetitive nature of Tera. You mention coverage moves, and I want to address this.
In order to prove that something is uncompetitive, we need to first define the word competitive.
A competitive game is one in which the better player wins through strategy and skill, and the worse player loses. The platonic ideal is chess. All players operate on complete information. The better player wins
100% of the time (not counting blunders).
Anything that detracts from this ratio is uncompetitive.
Some examples are pretty obvious. Critical hits and secondary effects are clearly uncompetitive, but we keep them because they're a part of what Pokemon is. For that, we're willing to sacrifice some of the competitiveness of the game.
What about random coverage moves to lure your opponent? Stone Edge on Zama? Energy Ball Jellicent? If you think about it, this is actually uncompetitive. That is why some VGC tournaments run open team sheets! We don't do open team sheets on Showdown because getting surprised can add to the excitement of the game and the use of niche lures is not so oppressive as to detract from peoples' enjoyment. Now, if there was an option for any mon to get an on-demand coverage move of their choice, that would be more uncompetitive. Wait a minute...
Finally, we get to Tera. Going through these examples, it's pretty clear that Tera is uncompetitive. You could draw a valid comparison to slotting random lure moves. Zama can click Tera Fire to turn the tables Moltres, just as it could click Stone Edge. Now it can also click Tera Dark to beat Ghold or Tera Steel to beat Fairies. When we add Tera into the equation, this is yet another layer, accessible to every mon, that increases the unpredictability of the game.
The devil's advocate point of view is that sometimes we make sacrifices to increase the fun of the game. Just as it's exciting to snipe your opponent's Moltres with a Stone Edge Zama, Tera adds a new layer of nuance to the game that is arguably fun, albeit decreasing the competitiveness. Let's face it, we're here to play Pokemon. If we only cared about playing the most competitive game, we'd be playing chess.
What's up for debate is the following: By how much does Tera decrease the game's competitiveness? And is the trade-off for excitement and fidelity to Pokemon worth it?
I do not know the answers to these questions. And there are many players more skilled and knowledgable than I who could weigh in on this.
What I do know is that we can and should continuously re-evaluate the controversial mechanics of this game. We did it with Sleep. I don't know if Tera should be banned, but it's not so sacrosanct as to be immune to reconsideration.