Hi there, I wanted to share my point of view on this.
The first thing I would like to point out is that Mega Sableye isn’t mandatory on stall. Far from it, and I would even go as far as to say that Mega Sableye is bad on stall.

The only reason stall uses Mega Sableye is because it helps to prevent hazards (mainly Stealth Rocks) being on the field.
But the thing is, Mega Sableye loses to every GOOD Stealth Rocks / Spikes user in SM metagame. Below are some calcs:

Landorus-T with Sword Dance:
+2 252+ Atk Earth Plate Landorus-Therian Earthquake vs. 252 HP / 252+ Def Sableye-Mega: 288-339 (94.7 - 111.5%) -- 68.8% chance to OHKO

Marowak-Alolan (not sure about the spread though):
136+ Atk Thick Club Marowak-Alola Shadow Bone vs. 252 HP / 252+ Def Sableye-Mega: 121-144 (39.8 - 47.3%) -- guaranteed 3HKO

Garchomp with SD:
+2 252 Atk Life Orb Garchomp Earthquake vs. 252 HP / 252+ Def Sableye-Mega: 263-309 (86.5 - 101.6%) -- 12.5% chance to OHKO
+2 252 Atk Garchomp Earthquake vs. 252 HP / 252+ Def Sableye-Mega: 202-238 (66.4 - 78.2%) -- guaranteed 2HKO

Spikes LO Greninja:
252 SpA Life Orb Greninja Hydro Pump vs. 252 HP / 4 SpD Sableye-Mega: 175-208 (57.5 - 68.4%) -- guaranteed 2HKO

LO Mamoswine:
252+ Atk Life Orb Mamoswine Earthquake vs. 252 HP / 252+ Def Sableye-Mega: 144-172 (47.3 - 56.5%) -- 82.8% chance to 2HKO

Heatran still wins versus Mega Sableye (I know it’s going to be trapped by Dugtrio but this is not my point there, remember)

Excadrill can use SR on Mega Sableye (same here)

Hippowdon, despite being not that great in SM OU, beats Mega Sableye (you have to be patient, though)

Seismitoad is another pokemon that can get SR up versus Mega Sableye

And I could even mention SR Clefable and SR Necrozma (with Dazzling Gleam), I know it’s not really viable now, but those pokemons can still SR versus Mega Sableye.

Also don’t forget some of those calcs do not even include Z moves which can get easier OHKOs.
This should make you aware that Mega Sableye only beats passive SR users such as Ferrothorn and Skarmory.
I mean, technically, even defensive Landorus-T can just U-Turn 3 times on Mega Sableye and then it is on the range of a 2HKO by Earthquake.

At that point, stall should use Double Defog rather than Mega Sableye. Well, I should just say Zapdos (+ Skarmory), since Zapdos beats the only two SR users Mega Sableye is able to beat, aka Ferrothorn and Skarm.
(And Zapdos has a ton of other uses on stall, so why even using Mega Sableye ? But well, this is a bit off topic.)


The idea behind this little talk is that Mega Sableye really does not prevent most SR users from setting up Stealth Rocks.


But let’s back to the crux of the matter, Dugtrio / Arena Trap:

I will start by listing the threats Dugtrio can remove. I remind the standard set used on stall runs Focus Sash + Earthquake + Reversal + Screech / Sucker Punch / Pursuit / Aerial Ace / Stone Edge / Rock Tomb:

- Hoopa-U
- Tapu Lele
- Mega Metagross
- Heatran
- Tyranitar
- Mega Charizard X
- Bisharp
- Kyurem B
- Magnezone
- Excadrill
- Jirachi
- Magearna
- Volcarona
- Crawdaunt
- Diggersby
- Weavile
- Terrakion
- Mega Gyarados
- Kabutops
- Omastar
- Kartana
- Mega Sharpedo
- Kingdra
- Mamoswine
- Muk-Alolan
- Nidoking
- Volcanion
- Xurkitree
- Chansey
- Toxapex
- others

And indeed, that’s A LOT of threats. But let’s be real, there are many threats here that do not really scare a good stall team, so we can reduce it to “what stall actually wants Dugtrio to remove”, and here it is:

- Hoopa-U
- Tapu Lele
- Mega Metagross
- Heatran
- Tyranitar
- Kyurem B
- Diggersby
- Crawdaunt

Yeah, this is basically what Dugtrio is usefull for. It just gives stall an answer to 8 pokemons that are otherwise really hard to deal with.
And still, stall can play around and has other answers than Dugtrio for some of them.

Because Dugtrio can beat the aforementioned threats doesn’t mean it automatically removes them, nor that it beats most stall breakers.
Those are two important things I woud like to highlight:

1)	Dugtrio can be beaten.
2)	There are still many answers to stall.


1)	How to beat Dugtrio ?

Remember, Dugtrio does not counter anything, it is only a RK.
That means it can only come on a double switch or thanks to an Eject Button.
Otherwise, stall has to lose a pokemon. You can take advantage of this.

In order to counter the threats it is supposed to beat, Dugtrio needs Entry Hazards to not be on field.

If SR are up, it means Dugtrio:
- has no sash,
- can’t switch in on anything,
- can’t use Reversal (thus it won’t be able to beat things like Kyurem B, Diggersby, …)
- can’t use double EQ which is needed to remove most threats (Tapu Lele, …),
- can’t RK faster threats / loses to priority (Bullet Punch, Aqua Jet, Quick Attack).

Basically, once EH are up, stall loses to almost any relevant wall/stall breaker. And Dugtrio won’t remove the threats it is supposed to beat anymore.

And as I said at the beginning, Mega Sableye is really not a big deal, Skarmory and Zapdos are what you need to be careful of.

Another way of beating Dugtrio is to force its sash to break with a first pokemon, and then you can capitalize on this with the proper wall breakers.
You could also use Pursuit for that if you manage to have Dugtrio on field.

Shed shell is also a viable option on some pokemons (see Tapu Lele).


Anyway, more generally, you’re not winning the stall match up with only one dedicated pokemon and without putting some pressure on.

For example if you want to wall break stall teams with Choice Band Diggersby, make sure you can have SR up in this match up, but don’t complain about Dugtrio because you didn’t prepare for it. Otherwise, well, just use an other wall breaker.
I don’t see how preparing for Chansey if you use a Charizard Y team is so different than preparing for Dugtrio if that pokemon can remove your breakers.
I mean, offense has to adapt to stall as much as stall has to adapt to offense.


2)	Answers to SM stall.

Now that I said everything I wanted to underline, here are some answers that break SM stall:

EH users (in addition with wall/stall breakers):
Landorus-T with SD + Stone Edge / Smack Down + Earthquake + SR + Rockium Z
Marowak-Alolan
Offensive Garchomp
Spikes Greninja (why is this great pokemon not used more ?)
Shed Shell Heatran
Seismitoad
Clefable / Necrozma (since most stall teams do not carry Zapdos lol)

Breakers:
Landorus-T with SD + Smack Down + Earthquake
Coil Toxic Zygarde with Rest or Substitute
Manaphy
SD Tapu Bulu with Fightinium Z
Shed Shell Tapu Lele
Taunt + Nature Madness Tapu Fini
SD Marowak-Alolan
Taunt Tapu Koko
Tornadus-T if no Zapdos
Togekiss stall breaker if no Zapdos
Growth Venusaur
Z Happy Hour Jirachi
Stored Power Espeon

Cores:
Gliscor + Magnezone
Knock Off Landorus-T + Magnezone + anything that needs Skarmory dead
Just be creative here !

Also, don’t forget Dugtrio can also work on offense.
Some cores such as Dugtrio + Volcarona or Dugtrio + Mega Charizard Y can easily remove Chansey so Volcarona / Charizard Y can sweep.

The SM metagame is still young, and I’m pretty sure there are unexplored things such as good Z move users.

I really think it is way too early to suspect Arena Trap / Dugtrio. The metagame is not even stable and part of the reason it seems so good is because most people don’t prepare for it the right way (or just don’t want to).