There is just so much wrong with this statement. You compared Shadow Tag with Arena Trap, which is not justifiable at all in Monotype as the users of Arena Trap and the users of Shadow Tag serve drastically different purposes. First of all, Arena Trap does NOT trap “almost as many mons as shadow tag does,” it traps threats that Ground as a type can already beat. Dugtrio’s main purpose comes from its ability to somewhat check Tapu Bulu with Sludge Wave. It is absolutely and in no level as bad as Shadow Tag.
subcm keldeo, choice specs keldeo, kyurem black, tapu bulu, manaphy, raichu-aloha, choiced locked mamoswine, nihilego (this can be dangerous with grass knot), infernape, etc. The combination of earthquake + sucker punch also takes out LO mamoswine after a small amount of prior damage. Dugtrio also has other options in the form of Toxic, screech, memento, and aerial ace. Dugtrio beats many more threats than "Ground as a type can already beat", and this quite frankly shows you know very little about dugtrio's role (and by extension, arena trap's) in the metagame.
Another mistake in your argument is saying STAG is “most effective vs defensive teams” while completely ignoring Wobbuffet’s role in Monotype as a Pokemon that can trap almost every single choice-locked Pokemon in the game. Defensive teams in Monotype hardly exist. The argument was that it dismantles defensive cores, not anything else. Saying “STAG traps things, and therefore is uncompetitive” is not an argument to be made, it’s just a fact. It falls under uncompetitive in the tier policy (tier policy literally brings up Shadow Tag as an example) so this argument is impossible to win.
As for your point on defensive teams, that was my point entirely: Goth (and by an extension, tag, as goth is tag's best abuser) works best vs stall. Because defensive teams hardly exist in monotype, goth loses its best matchup. The question is if STAG is "noncompetitive" vs offense and balance. This is clearly not "just a fact", as you claim, as the tier leaders have chosen to have a suspect instead of quickbanning it.
How is goth vs steel an unfair portrayal? Once Heatran is trapped, the game is basically over. Victini lacks any switch-ins outside of Heatran and can even live Bisharp’s Sucker Punch.
Also, with the standard sets, provided by pokemon calculator:
252+ Atk Life Orb Bisharp (Monotype swords dance (steel)) Sucker Punch vs. 0 HP / 0- Def Victini (Monotype choice scarf (Psychic)): 354-421 (103.8 - 123.4%) -- guaranteed OHKO
Of course one can always opt to not use a tini with a -def nature, but even so tini dies more often than not:
252+ Atk Life Orb Bisharp Sucker Punch vs. 0 HP / 0 Def Victini: 320-377 (93.8 - 110.5%) -- 62.5% chance to OHKO
Actually trapping it is extremely easy and you’re making it seem unnecessarily difficult. You mentioned it yourself, spamming U-turn with Victini until Heatran comes in is a perfectly viable play to make. Your balloon point is not relevant as Victini’s same u-turn from your previous example will always pop it. Heatran can’t even afford to stay in on specially defensive Jirachi, as it will always live a hit from Heatran and proceed to pivot into Gothitelle who can trap and kill it.
The point of the air balloon was that, if heatran comes in as victini uses a fire STAB move, tini has to switch out (as it's choiced) and heatran gets to fire off a flash-fire boosted magma storm. Then, even if heatran gets a kill that turn, goth can't come in and trap it as heatran will still be holding an air balloon.
This is the best point you manage to make during your entire post, and it’s absolutely true. Not only is Goth a liability after it traps it’s intended target, it’s probably dead or extremely weak. Gothitelle can, however, choose exactly what it wants to trap in this matchup, whether it’s Mantine or Skarmory. This is a very unfair trade in the opponent’s part as the player can simply pick and choose what to trap and identify a wincon they can sweep in later in the game.
I'm not entirely sure if this trade is good for the psychic user.
Offensive Flying is hardly as common or viable as balanced Flying, so I’m not sure why bring that up.
You lost me there. This is not a good set in Monotype. Please lurk a bit more and familiarize yourself with the metagame before posting about Shadow Tag.
252 Atk Victini V-create vs. 252 HP / 212+ Def Thick Fat Venusaur-Mega: 158-188 (43.4 - 51.6%) -- 99.6% chance to 2HKO after Stealth Rock --no switchins
Not to mention megavior, z mew, etc, are all also very scary threats to grass in their own right.
Vs Flying, Gothitelle traps the intended target and removes it. It cannot possibly be considered a liability if you get to erase your target from the match. This would be like saying on Poison teams, Alolan Muk is a liability after it traps and kills something like Alolan Raichu on Electric; the entire point of the mon is to erase that specific threat, and once completed, it can be fodder for the rest of the match and still have pulled its own weight.
Neither Mantine nor skarmory are essential to preserve for balanced flying in this matchup, as mega gardevior is able to break through both rather easily with psyshock and focus blast, and so can lati@s (with tbolt), tini (with bolt strike), alakazam (with psyshock/focus blast), etc. As goth switches in, the flying user defogs/sets up hazards, while after skarm/maintine die, the flying user gains free momentum on a choiced tbolt.
I'm not entirely sure if this trade is good for the psychic user.
When you say "Goth has to actually trap the threat", it's not exactly hard for Goth to switch into something like Chansey and trick a scarf onto it. Goth isn't going to be trapping mons like CB Heracross locked into Megahorn; the targets it is tailored to trap aren't exactly going to beat it 1v1...
252+ Atk Choice Band Huge Power Diggersby Return vs. 252 HP / 232+ Def Slowbro: 220-261 (55.8 - 66.2%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Choice Band Huge Power Diggersby Return vs. 252 HP / 0 Def Jirachi: 161-190 (39.8 - 47%) -- guaranteed 3HKO after Leftovers recovery--even after scouting for return the specially defensive variant still take tons of damage, and is forced to wish/tect to avoid being koed, giving the normal user a free switch
252+ Atk Choice Band Huge Power Diggersby Return vs. 0 HP / 0 Def Jirachi: 161-190 (47.2 - 55.7%) -- 78.5% chance to 2HKO--scarfrachi can't switch in on return more than once
Goth eliminating Mandibuzz in the Psy vs. Dark matchup is actually huge as it allows physical threats like Victini and Mega-Gallade to have a much easier shot in sweeping,