Also, top-tier Pokemon exist. You acknowledged that by listing Drapion in the context of a lower-tier Pokemon and Blaziken as a top-tier one. Obviously, those top-tier Pokemon have counters. But that doesn't mean that, in most other situations, Blaziken or Starmie won't be more helpful than Drapion and Klingklang. Compare a lineup with Blaziken, Starmie, Salamence, Gengar, Alakazam, and Jolteon to a lineup of Gengar, Venusaur, Drapion, Crobat, Tentacruel, and Nidoking. On base stats and variety of movepool alone, those top-tier Pokemon easily trump the Poison-types (which can be considered to be some of the better Pokemon, in general at least, of their type). That's an advantage Poison-types, or no single-type gym, can come close to, because they will always be limited to, at best, the "top" four/six Pokemon of their type in regards to base stats/movepool.
Obviously, a good user with a bad team will beat a bad user with a good team, but between two equal users where one has a good team and a bad team, the better team usually wins. Multitype TED vs Flying-type Pidge is different from Ground-type TED and Flying-type Pidge.