Trapping everything means nothing if you cant actually beat the pokemon, diglett right here traps more pokemon than magnezone or pursuit ever did but it means nothing if all it does is get 30-60% off and dieComparing Arena Trap in any capacity to Magnet Pull is a farce because the scope of what they trap is absolutely incomparable. Magnet Pull, as peng described, literally only works on one type, which already makes it infinitely easier to prep for since you can react accordingly upon seeing a Magnezone or just slap on a Shed Shell or some other counter tech in the teambuilder. On the other hand, Arena Trap works on everything but two types + levitators. There is no conceivable way you can do Magnezone-style counter-teaming with every single suspectible Pokemon without gimping your team. In addition, 2 out of the 3 things that are immune to Arena Trap would beat Dugtrio anyway if it had any other ability. Mandibuzz, Corviknight, Rotom-Wash and so on were never gonna lose unless Dugtrio just so happened to have Mold Breaker or Huge Power or something. What this does is centralize the meta around Arena Trap and its few innate checks, which with the exception of the likes of Corviknight aren't infallible: Flyings are weak to Stone Edge, there are only a handful of viable Levitators and two of the 3 main ghosts get fricced by STAB EQ anyway even if they can switch out. An ability which eliminates an essential option for 6v6 play - the act of switching - from 88% of the type chart and Levitators has no place in a competitive environment.
88% of the type chart is hugely exaggerated when realistically the only pokemon getting consistently trapped are steel, rock, poison and electric types
I dont disagree on dugtrio being a problem at all, but stop acting like arena trap is synonymous to dugtrio because its not the same