Your argument holds water if Diglett is actually vulnerable to other setup sweepers. But it isn't. In fact the only things it fears are: Snivy, Corphish, Carvanha, Tirtouga (a 50/50 essentially), Pawniard (again a 50/50), 14 Speed Scarvers, Fletchling, and various little mons that are all rolls (namely Zigzagoon). Every other Pokemon is simply bopped with Memento and unable to actually do anything. Agility Bunnelby isn't exactly scary when its -2 Attack.
Except you are forced to use Ponyta / Larvesta / Trubbish etc as your mienfoo counter. Your only other option are things like Spritzee or other various bulkymons. And even then they're easily worn down and trapped and killed.
Looking at the list of viable Pokemon that counter Mienfoo alone, from top to bottom: Abra, Spritzee, lolevio Archen, Ponyta, Snubbull, Vullaby, Cottonee, Foongus, Larvesta, Skrelp, Hippopotas, Croagunk, Tentacool, Koffing, Shellos, Trubbish
Half of those aren't even counters, because they simply get Knock Off'd and U-turned on (namely things like Snubbull, Tentacool, Croagunk, Hippopotas, etc). Abra, Ponyta, Snubbull, Larvesta, Skrelp, Croagunk, Tentacool, Trubbish are all trapped and killed by Diglett. At best Spritzee gets U-turned on, worst loses its Eviolite. Evio Archen is kinda a bad set, and it can only switch in once on HJK, and even then it goes into Defeatist from HJK + Rocks, forcing it into a Roost loop (which mienfoo wins either way, it wins the speed tie and 2hkoes or it kills anyway from Roost getting rid of Flying-type). Cottonee isn't exactly a threat to anything, and certainly won't be killing anything ever. Foongus is a good check to it, but fears Acrobatics Foo and once it loses its eviolite its easily koed by Diglett which deals 64% (16 HP) minimum to it, meaning U-turn + EQ wins. Snubbull is killed anyway by U-turn+EQ unless you happen to get a low roll on both U-turn+EQ, not have rocks up, and have 0 attack Mienfoo. Vullaby is 2hkoed by Scarffoo HJK + rocks (and even sometimes with rocks), and thats assuming that vullaby is full health with eviolite still in tact. So its only a soft counter that can switch into certain sets. Oh yeah, it also loses to stone edge. Hippopotas is easily worn down and set up bait for things like Fletchling depending on the set. Its also very momentum killing.
But yeah, you aren't forced to use Larvesta / Ponyta / Trubbish. You can certainly use Spritzee / Cottonee / Koffing / Shellos. Or you can use the 11 other Pokemon that just flat out lose to a Mienfoo with attack investment+Diglett.
An even further example of just how bullshit Diglett is in combination with certain Pokemon is Fletchling, which only "wins" if you use Archen / winning the 50/50 with Pawniard, Scarf Chou, Vullaby, Hippopotas, Aqua Jet Tirtouga, BJ Elekid + winning the speed tie, Endure Magnemite, Lickitung, Slowpoke (relies on a Scald burn or timely paralysis), or lileep. Which you can certainly do. But theres a reason half of those Pokemon in both sets are B- or below in viability rankings: they simply suck compared to A rank. They are a shitton harder to fit on a team that can meet the same standard of quality as fletchling / diglett / mienfoo / pawniard / archen / chinchou or whatever you want to run, a lot harder to actually use without just straight up sucking momentum away, and they are in general one trick mons, where half the time they can't even "counter" they simply can status something or set up SR (Cottonee, Hippopotas, Lileep). In short, no, we aren't "forced" to use x / y / z, but when the alternatives are absolute dogshit to use outside of one specific scenario, something is bloody wrong.
Your argument further down is even more dogshit spewing from your keyboard. Zigzagoon+Pursuit user don't even begin to compare to Mienfoo+Diglett, or Fletch+Diglett, or any other variation. Zigzagoon is a high risk medium reward pokemon, that absolutely needs its dick sucked 24 / 7 throughout the match, and have literally every single thing in a frame for it to even do any work. Its so fucking pointless, the amount of work required to set up a zigzagoon sweep, that you may as well not even use zigzagoon, since by the time zig can sweep literally any other Pokemon with half the previous effort would already have won. Zigzagoon needs all ghost-types completely gone, all steel-types, namely Pawniard+Magnemite, completely gone, needs to have the perfect set up opportunity in nothing that can OHKO it (which is a large portion of the tier), and needs to not be statused at all, so neither burned nor para'd nor slept by things like Snubbull, Foongus, Ponyta, Larvesta, TWave Chinchou, various Scald users. It also needs most Rock-types gone, like Tirtouga and Archen. It literally requires you to give it a handy, wet it a little, finger a little, pay for dinner, and then doesn't even go all the way because of how stupid it is. A zigzagoon sweep is the most painfully obvious strategy there is, and fails to compare to Diglett in any way. Any player worth their salt can spot a zigzagoon oriented team a mile away, even if they had no preview. So no, its not Zigzagoon+Pursuit user. Its, Cottonee+Diglett+Mienfoo+Zigzagoon+Stunky+Filler. Substitute Mienfoo for another Fighting-type and Stunky for a different Pursuiter (idk why but w/e) and the team is basically the same, only slightly different. They are NOT the same at all in any way.