Defeatist hasn't just prevented Archeops from being overpowered, it has turned Archeops almost into a non-threat, due to its excessive frailty and vulnerability to many super effective attacks. It also discourages the use of Archeops' most powerful attack, Head Smash. Sadly, it is the only ability Archeops can obtain, and while it is difficult to work around, doesn't make Archeops entirely neutered.
Source: Smogon/ Defeatist
There really is no way to get around it except with Skill Swap or Mummy, but how many Cofagrigus will you see?? It is too situational. And Skill Swap, by the time you get to do that, he might be dead.